scripts: add scripts/gnu_make to not hard-code make command

U-Boot is expected to be built on various platforms
but makefiles are written for GNU Make.

We should keep in mind that the command 'make' is not always 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.

Instead, they should call the make command via this helper script
to make sure it is 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>
Cover-letter:
Do not hard-code the command name 'make' in upper-level tools

END
MAKEALL: make sure to invoke GNU Make

Since the command name 'make' is not GNU Make on some platforms
such as FreeBSD, MAKEALL should call the make via scripts/gnu_make.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
buildman: make sure to invoke GNU Make

Since the command name 'make' is not GNU Make on some platforms
such as FreeBSD, MAKEALL should call the make via scripts/gnu_make.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>