arch: toolchain: Introduce binary format FLAT types.

Just introduce the symbol and options in arch generic Config.in.
Add FLAT types specific compiling flags into package makefile.

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>