arch: tidy up binary formats config

Instead of (black)listing architectures when deciding the binary format,
we can enable the ELF format only when using an MMU and FLAT only when
we're not. This mimics the logic in the Linux kernel for user binaries
support.

For FDPIC, we introduce a Kconfig option to enable its selection, and
have blackfin select it.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>