libfdt requires memchr. Add missing function to libc. Change-Id: I872026559d16a352f350147c9d7c4be97456a99f Signed-off-by: Philipp Hug <philipp@hug.cx> Reviewed-on: https://review.coreboot.org/c/31354 Tested-by: build bot (Jenkins) <no-reply@coreboot.org> Reviewed-by: Julius Werner <jwerner@chromium.org>