arch: define dependencies for the binfmt flat formats

arch: define dependencies for the binfmt flat formats

The situation looks like following for elf2flt and binfmt FLAT:

 * Only gcc for bfin/m68k implements
   -msep-data (BR2_BINFMT_FLAT_SEP_DATA) and
   -mid-shared-library (BR2_BINFMT_FLAT_SHARED), so the corresponding
   options are made only visible on those architectures.

 * When the default of BR2_BINFMT_FLAT_ONE is used on m68k, broken
   binaries are produced, which mainly end up in SIGILL, so do not use
   it for m68k.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
[Thomas:
 - also add the dependencies on m68k/bfin to BR2_BINFMT_FLAT_SHARED
 - rework commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>