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>