region: Introduce region_create() functions

We introduce two new functions to create region objects. They allow us
to check for integer overflows (region_create_untrusted()) or assert
their absence (region_create()).

This fixes potential overflows in region_overlap() checks in SMI
handlers, where we would wrongfully report MMIO as *not* overlapping
SMRAM.

Also, two cases of strtol() in parse_region() (cbfstool),  where the
results were implicitly converted to `size_t`, are replaced with the
unsigned strtoul().

FIT payload support is left out, as it doesn't use the region API
(only the struct).

Change-Id: I4ae3e6274c981c9ab4fb1263c2a72fa68ef1c32b
Ticket: https://ticket.coreboot.org/issues/522
Found-by: Vadim Zaliva <lord@digamma.ai>
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79905
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/lib/fmap.c

@@ -199,8 +199,7 @@ int fmap_locate_area(const char *name, struct region *ar)
 		printk(BIOS_DEBUG, "FMAP: area %s found @ %x (%d bytes)\n",
 		       name, le32toh(area->offset), le32toh(area->size));
 
-		ar->offset = le32toh(area->offset);
-		ar->size = le32toh(area->size);
+		*ar = region_create(le32toh(area->offset), le32toh(area->size));
 
 		rdev_munmap(&fmrd, area);