mkimage: make mmap() checks consistent

The mmap() related code is full of inconsistent casts/constants when
it comes to error checking, and may break when building on some
systems (like ones that do not implicitly define the caddr_t type).
Let's just avoid the whole mess by writing the code nice and clean in
the first place.

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