efivar: introduce BR2_PACKAGE_EFIVAR_ARCH_SUPPORTS

efivar: introduce BR2_PACKAGE_EFIVAR_ARCH_SUPPORTS

efivar only makes sense on platforms that support UEFI.
UEFI is only supported by some architectures at the moment, being mostly
employed on platforms such as x86, x86_64 and aarch64. Some other
platforms such as MIPS and PowerPC may have some unofficial UEFI
support. UEFI is also limited to little endian architectures.

efivar was being supported in Buildroot without architecture
restrictions so far, however this has led to the creation of a number of
hacks in the recipes, mostly for architectures that are not supported by
UEFI.
In order to avoid spending more time to debug these failures and
maintaining more hacks for unsupported architectures, efivar can be
restricted to that platforms where it makes sense and where it is more
likely to receive some testing and actual usage.

The existing hacks for the now unsupported architectures are removed,
and the dependency is propagated to efibootmgr as it depends on efivar.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>