arch: toolchain: Introduce binary formats BINFMT_*.

Just introduce the symbol and options in arch generic Config.in.
Append FLAT format link flags to external toolchain wrapper.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>