Build: Enable ninja-based builds

This commit enables to use of `-GNinja` for builds, which
provides significantly faster TF-M builds, and enables
building TF-M as a cmake external project with Zephyr's
build system.

```
$ cmake -GNinja \
  -DPROJ_CONFIG=`readlink -f ../configs/ConfigDefault.cmake` \
  -DTARGET_PLATFORM=LPC55S69 -DBL2=False -DCOMPILER=GNUARM ..
$ ninja
```

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>
Change-Id: I5deaaf9ffb3ec3329174881d70fe7033b531ec49
diff --git a/cmake/Common/CompilerGNUARMCommon.cmake b/cmake/Common/CompilerGNUARMCommon.cmake
index d616a2d..6d427ec 100644
--- a/cmake/Common/CompilerGNUARMCommon.cmake
+++ b/cmake/Common/CompilerGNUARMCommon.cmake
@@ -146,6 +146,7 @@
 	set_property(TARGET ${TARGET} APPEND_STRING PROPERTY LINK_FLAGS " -Wl,--cmse-implib,--out-implib=${FILE_PATH}")
 	#Tell cmake cmse output is a generated object file.
 	SET_SOURCE_FILES_PROPERTIES("${FILE_PATH}" PROPERTIES EXTERNAL_OBJECT true GENERATED true)
+	add_custom_command(TARGET ${TARGET} POST_BUILD COMMAND ${CMAKE_COMMAND} -E echo "" BYPRODUCTS ${FILE_PATH})
 	#Tell cmake cmse output shall be removed by clean target.
 	get_directory_property(_ADDITIONAL_MAKE_CLEAN_FILES DIRECTORY "./" ADDITIONAL_MAKE_CLEAN_FILES)
 	set_directory_properties(PROPERTY ADDITIONAL_MAKE_CLEAN_FILES "${_ADDITIONAL_MAKE_CLEAN_FILES} ${FILE_PATH}")