make BINFMT_FLAT a bool

I have not yet seen anyone saying he has a reasonable use case for using
BINFMT_FLAT modular on his embedded device.

Considering that fs/binfmt_flat.c even lacks a MODULE_LICENSE() I really doubt
there is any, and this patch therefore makes BINFMT_FLAT a bool.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Bryan Wu <cooloney.lkml@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>