Rename tools/Makefile.* to have the .mk extension

This PR renames `tools/Makefile.*` to have the `.mk` extension. This PR also updates `README.md` and other files that references the other files.
Note: Skipped Makefile.host for this PR since it caused failures in CI for the sim build

By using a standard extension for Makefiles (https://www.file-extension.info/format/mk), editors will auto-format files.
This change will also improve developer ergonomics when searching for specific files

Verified locally that build still works, CI will verify more!

tools/Export.mk

@@ -1,5 +1,5 @@
 ############################################################################
-# tools/Makefile.export
+# tools/Export.mk
 #
 # Licensed to the Apache Software Foundation (ASF) under one or more
 # contributor license agreements.  See the NOTICE file distributed with

tools/README.txt

@@ -175,11 +175,11 @@ lowhex.c
 Makefile.[unix|win]
 -----------------
 
-  Makefile.unix is the Makefile used when building NuttX in Unix-like
-  systems.  It is selected from the top-level Makefile.
+  Unix.mk is the Makefile used when building NuttX in Unix-like systems.
+  It is selected from the top-level Makefile.
 
-  Makefile.win is the Makefile used when building natively under
-  Windows.  It is selected from the top-level Makefile.
+  Win.mk is the Makefile used when building natively under Windows.
+  It is selected from the top-level Makefile.
 
 mkconfig.c, cfgdefine.c, and cfgdefine.h
 ----------------------------------------
@@ -191,10 +191,10 @@ mkconfig.c, cfgdefine.c, and cfgdefine.h
   in the top level NuttX directory (See boards/README.txt or
   Documentation/NuttXPortingGuide.html).  The first time you make NuttX,
   the top-level makefile will build the mkconfig executable from mkconfig.c
-  (using Makefile.host).  The top-level Makefile will then execute the
-  mkconfig program to convert the .config file in the top level directory
-  into include/nuttx/config.h.  config.h is a another version of the
-  NuttX configuration that can be included by C files.
+  (using Makefile.host).  The top-level Makefile will then execute the mkconfig
+  program to convert the .config file in the top level directory into
+  include/nuttx/config.h.  config.h is a another version of the NuttX
+  configuration that can be included by C files.
 
 mkconfigvars.sh
 ---------------
@@ -218,14 +218,14 @@ mkconfigvars.sh
     -h
        show this help message and exit
 
-mkexport.sh and Makefile.export
+mkexport.sh and Export.mk
 -------------------------------
 
   These implement part of the top-level Makefile's 'export' target.  That
   target will bundle up all of the NuttX libraries, header files, and the
   startup object into an export-able, binary NuttX distribution.  The
-  Makefile.export is used only by the mkexport.sh script to parse out
-  options from the top-level Make.defs file.
+  Export.mk is used only by the mkexport.sh script to parse out options
+  from the top-level Make.defs file.
 
   USAGE: tools/mkexport.sh [-d] [-z] [-u] -t <top-dir> [-x <lib-ext>] -l "lib1 [lib2 [lib3 ...]]"
 
@@ -253,8 +253,8 @@ mkversion.c, cfgdefine.c, and cfgdefine.h
   When you build NuttX there should be a version file called .version in
   the top level NuttX directory (See Documentation/NuttXPortingGuide.html).
   The first time you make NuttX, the top-level makefile will build the
-  mkversion executable from mkversion.c (using Makefile.host).  The top-
-  level Makefile will then execute the mkversion program to convert the
+  mkversion executable from mkversion.c (using Makefile.host).  The top-level
+  Makefile will then execute the mkversion program to convert the
   .version file in the top level directory into include/nuttx/version.h.
   version.h provides version information that can be included by C files.
 

tools/Unix.mk

@@ -1,5 +1,5 @@
 ############################################################################
-# tools/Makefile.unix
+# tools/Unix.mk
 #
 # Licensed to the Apache Software Foundation (ASF) under one or more
 # contributor license agreements.  See the NOTICE file distributed with

tools/Win.mk

@@ -1,5 +1,5 @@
 ############################################################################
-# tools/Makefile.win
+# tools/Win.mk
 #
 # Licensed to the Apache Software Foundation (ASF) under one or more
 # contributor license agreements.  See the NOTICE file distributed with

tools/mkexport.sh

@@ -163,7 +163,7 @@ cp -a "${TOPDIR}/Make.defs" "${EXPORTDIR}/Make.defs" ||
 
 # Extract information from the Make.defs file.  A Makefile can do this best
 
-${MAKE} -C "${TOPDIR}/tools" -f Makefile.export TOPDIR="${TOPDIR}" EXPORTDIR="${EXPORTDIR}"
+${MAKE} -C "${TOPDIR}/tools" -f Export.mk TOPDIR="${TOPDIR}" EXPORTDIR="${EXPORTDIR}"
 source "${EXPORTDIR}/makeinfo.sh"
 rm -f "${EXPORTDIR}/makeinfo.sh"
 rm -f "${EXPORTDIR}/Make.defs"