Since the command name 'make' may not be GNU Make on some platforms
such as FreeBSD, buildman should call scripts/show-gnu-make to get
the command name for GNU MAKE (and error out if it is not found).

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Jeroen Hofstee <jeroen@myspectrum.nl>