binutils: disallow selection of 2.27 on ARM/noMMU

binutils: disallow selection of 2.27 on ARM/noMMU

binutils 2.27 triggers a segfault in elf2flt on ARM/noMMU. While Arnout
has identified a binutils 2.28 commit that can be backported on 2.27,
this commit is huge and we don't clearly understand the impact.

Since both binutils 2.26 and 2.28 are unaffected by this issue, we
simply disallow the selection of binutils 2.27 on ARM/noMMU, and default
to binutils 2.28.

Fixes:

  http://autobuild.buildroot.net/results/e14cadb290b0b86cac12c4bfb681eb6eee9e6dea/
  and lots of other similar ARM/Cortex-M4 failures

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>