binfmt_flat: replace flat_argvp_envp_on_stack with a Kconfig variable

binfmt_flat: replace flat_argvp_envp_on_stack with a Kconfig variable

This will eventually allow us to kill the need for an <asm/flat.h> for
many cases.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com>
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>