boards: arm: Update README.txt regarding CONFIG_BUILD_KERNEL=y

Summary:
- This commit updates README.txt regarding

Impact:
- CONFIG_BUILD_KERNEL=y

Testing:
- build only

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

boards/arm/imx6/sabre-6quad/README.txt

@@ -913,7 +913,7 @@ Configuration sub-directories
     $ make V=1 -j7
     $ make export V=1
     $ cd ../apps
-    $ ./tools/mkimport.sh -x ../nuttx/nuttx-export-*.zip
+    $ ./tools/mkimport.sh -z -x ../nuttx/nuttx-export-*.tar.gz
     $ make import V=1
     $ cd ../nuttx
     $ qemu-system-arm -semihosting -M sabrelite -m 1024 -smp 4 -nographic -kernel ./nuttx
@@ -950,7 +950,7 @@ Configuration sub-directories
     $ make
     $ make export
     $ cd ../apps
-    $ ./tools/mkimport.sh -x ../nuttx/nuttx-export-*.zip
+    $ ./tools/mkimport.sh -z -x ../nuttx/nuttx-export-*.tar.gz
     $ make import
     $ cd ../nuttx
     $ qemu-system-arm -semihosting -M sabrelite -m 1024 -smp 4 -nographic -kernel ./nuttx

boards/arm/sama5/sama5d4-ek/README.txt

@@ -4114,7 +4114,7 @@ Configurations
         C. Build the file system image at apps/bin
 
         $ cd apps/                          : Go to the apps/ directory
-        $ tools/mkimport.sh -x <zip-file>   : Use the full path to nuttx-export-*.zip
+        $ tools/mkimport.sh -z -x <tgz-file>: Use the full path to nuttx-export-*.tar.gz
         $ make import                       : This will build the file system.
 
       You will then need to copy the files from apps/bin to an SD card or USB
@@ -4149,7 +4149,7 @@ Configurations
         C. Build the file system image at apps/bin
 
         $ cd apps/                          : Go to the apps/ directory
-        $ tools/mkimport.sh -x <zip-file>   : Use the full path to nuttx-export-*.zip
+        $ tools/mkimport.sh -z -x <tgz-file>: Use the full path to nuttx-export-*.tar.gz
         $ make import                       : This will build the file system
 
         D. Create the ROMFS file system image