scripts: add scripts/show-gnu-make to get GNU Make command name

U-Boot is expected to be built on various platforms.

We should keep in mind that the command 'make' is not always GNU Make,
while all the makefiles are written for GNU Make.

For example, on Linux, people generally do:

  make <board>_config; make

But FreeBSD folks do

  gmake <board>_config; gmake

(The command 'make' on FreeBSD is BSD Make, not GNU Make)

It is not a good idea to hard-code the command name 'make'
in MAKEALL or buildman.

They should call this helper script and get the command name
for GNU Make.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Series-to: uboot
Series-cc: sjg
Series-cc: trini
Series-cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Series-version: 2
Series-changes: 2
  - Use dashes for filename
  - Show the command name instead of invoking make.
    It allows us to check the command name once at startup
    (and error out if no GNU Make is found.)

Cover-letter:
Do not hard-code the command name 'make' in higher-level tools

END
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 processing options because
we want to allow to run "MAKEALL -h" even if GNU Make is missing
on the system.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Series-changes: 2
  - Check GNU Make only once at startup.
    It seems more reasonable because MAKEALL generally invokes
    make over and over again.
buildman: make sure to invoke GNU Make

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>
Series-changes: 2
  - Check GNU Make only once at startup.
    It seems more reasonable because MAKEALL generally invokes
    make over and over again.