arch: ensure BR2_BINFMT_ELF is really used

Currently, the binary format choice is only shown for the Blackfin and
m68k architectures, since we assume that all other architectures are
using the ELF binary format. However, due to this, the BR2_BINFMT_ELF
symbol is in fact not set to 'y' for those architectures that use the
ELF format.

This will be causing problems for the refactoring of the static/shared
library support, as we will need to know if the binary format supports
shared libraries or not.

Therefore, we simply make the choice visible on all architectures,
even if it means that on many architectures no other choice than ELF
will be available.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>