mboot: move map initialization out of map_image()

mboot: move map initialization out of map_image()

The amap/mmap initialization were buried randomly inside map_image(),
which at the very least makes the code needlessly hard to read.
Furthermore, it is at least possible that we may want to be able to
map multiple images in the future.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>