Makefile: Add filelist to help

The filelist target has been around for a while, but was never added
to the help.  Add it now.

Change-Id: Idc24ecc8028fc61c6b798c0c81431b05dfa25b0e
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/23472
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

Makefile

@@ -98,6 +98,7 @@ help_coreboot help::
 	@echo  '  distclean             - Remove build artifacts and config files'
 	@echo  '  doxygen               - Build doxygen documentation for coreboot'
 	@echo  '  doxyplatform          - Build doxygen documentation for the current platform'
+	@echo  '  filelist              - Show files used in current build'
 	@echo  '  printall              - print makefile info for debugging'
 	@echo  '  gitconfig             - set up git to submit patches to coreboot'
 	@echo  '  ctags / ctags-project - make ctags file for all of coreboot or current board'