Update Doxyfile to Doxygen 1.8.15

Generate Doxygen output in doc and ignore this directory in Git.  Add
RTEMS logo.  The Doxygen documentation is now built using the source
tree.  Just invoke "doxygen" in the top-level source directory.

The Doxyfile works also with at least Doxygen 1.8.13 and Doxygen 1.8.14.

Update #3705.

.gitignore

@@ -3,3 +3,4 @@ autom4te.cache
 configure
 config.h.in
 Makefile.in
+doc

cpukit/configure.ac

@@ -377,8 +377,6 @@ AM_CONDITIONAL([CPU_V850],[test $RTEMS_CPU = "v850"])
 AM_CONDITIONAL([CPU_X86_64],[test $RTEMS_CPU = "x86_64"])
 
 # Explicitly list all Makefiles here
-AC_CONFIG_FILES([
-Doxyfile
-Makefile])
+AC_CONFIG_FILES([Makefile])
 
 AC_OUTPUT