MAKEALL: make sure to invoke GNU Make

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

The GNU Make should be searched after parsing options because we want
to allow "MAKEALL -h" even if GNU Make is missing on the system.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>