package: Introduce package-specific BINFMT_FLAT options.

Add new option <PKG>_FLAT_STACKSIZE. The document needs to be updated.

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>