disable security warning flags when possible

Some toolchains enable security warning flags by default, but these don't
really make sense in the u-boot world.  Such as forcing changes like:
	-printf(foo);
	+printf("%s", foo);

So disable the flags when the compiler supports them.  Linux has already
merged a similar change in their build system.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>