cmake: util: Add fmerge.cmake script

In order to merge multiple files into a single one, this convenience
script is provided to be able to do so at build time in a manner
compatible with all operating systems.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

cmake/util/fmerge.cmake

@@ -0,0 +1,17 @@
+# Merges a list of files into a destination file.
+# Usage: list of files as arguments, first argument is the destination file
+
+MATH(EXPR ARGC "${CMAKE_ARGC}-1")
+# First 3 arguments are "cmake", "-P", and "process.cmake"
+if( ${CMAKE_ARGC} LESS 5)
+  message(FATAL_ERROR "Not enough arguments")
+endif()
+
+set(DEST_FILE ${CMAKE_ARGV3})
+# Empty the file
+file(REMOVE ${DEST_FILE})
+
+foreach(i RANGE 4 ${ARGC})
+	file(READ ${CMAKE_ARGV${i}} BUF)
+	file(APPEND ${DEST_FILE} ${BUF})
+endforeach()