bin_romfs.cmake: Only copy the contents of the input directory

Do not copy the input directory again, if the target directory already
exists. This confuses Ninja as it tries to remove the boot/ directory, but
it contains sub-directories the build system does not know about (it should
use "rm -rf" but it doesn't know it should)

platforms/nuttx/cmake/bin_romfs.cmake

@@ -86,7 +86,7 @@ function(make_bin_romfs)
 		# Preserve the files with debug symbols
 
 		add_custom_target(debug_${OUTPREFIX}
-			COMMAND cp -r ${BINDIR} ${BINDIR}_debug
+			COMMAND cp -r ${BINDIR}/. ${BINDIR}_debug
 			DEPENDS ${DEPS}
 		)