boot/grub2/Config.in: add symbols to represent legacy and EFI boot

There are cases to want a synthetic information whether the legacy BIOS
or U-Boot boot scheme, or the EFI boot scheme, are enabled, without
resorting to testing all and each platforms.

This is already the cae in grub2 itself, for the configuration of the
BIOS/U-Boot boot partition, and builtin modules and configuration on one
hand, and the EFI builtin modules and configuraiton on the other hand.

It is also the case for mender-grubenv, which will want to know if
either or both are enabled, but without having to resort to testing all
the cases.

Add two new symbols, that each represent those conditions:
  * BR2_TARGET_GRUB2_HAS_LEGACY_BOOT
  * BR2_TARGET_GRUB2_HAS_EFI_BOOT

Each target selects the appropriate bool, which makes it much more
simple for other packages such as mender-grubenv to check if grub legacy
or EFI is selected.

And of course, we also make use of those symbols in grub2 itself, to
simplify the conditions for showing.hiding legacy and EFI options.

Additionally (but that does not merit being in its own patch), add a
comment on the closing 'endif' for the EFI part.

Signed-off-by: Adam Duskett <aduskett@gmail.com>
[yann.morin.1998@free.fr:
  - s/BR2_TARGET_GRUB_/BR2_TARGET_GRUB2_/
  - rename variables anyway
  - use variables in grub2 itself
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Köry Maincent <kory.maincent@bootlin.com>