boot/grub2: add support to build multiple Grub2 configurations in the same build

When Grub2 is build it is configured only for one boot set-up, BIOS Legacy,
EFI 32 bit or EFI 64 bit. It can not deal with several boot set-up on the
same image.

This patch allows to build Grub2 for different configurations simultaneously.
To cover Grub2 configuration of legacy BIOS platforms (32-bit), 32-bit EFI
BIOS and 64-bit EFI BIOS in the same build, multi-build system felt much more
reasonable to just extend the grub2 package into 3 packages.

We can no longer use autotools-package as a consequence of this multi-build, and
we have to resort to generic-package and a partial duplication of
the autotools-infra. Grub2 was already using custom option like --prefix or
--exec-prefix so this won't add much more weirdness.

We use a GRUB2_TUPLES list to describe all the configurations selected.
For each boot case described in the GRUB2_TUPLES list, it configures and
builds Grub2 in a separate folder named build-$(tuple).
We use a foreach loop to make actions on each tuple selected.

We have to separate the BR2_TARGET_GRUB2_BUILTIN_MODULES and the
BR2_TARGET_GRUB2_BUILTIN_CONFIG for each BIOS or EFI boot cases.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
[yann.morin.1998@free.fr:
  - keep sub-options properly indented
  - fix check-package
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>