Restore the ability to continue booting after legacy image overwrite

Before new uImage code was merged, bootm code allowed for the kernel image to
get overwritten during decompresion. new uImage introduced a check for image
overwrites and refused to boot the image that got overwritten. This patch
restores the old behavior. It also adds a warning when the image overwriten is
a multi-image file, because in such case accessing componentes other than the
first one will fail.

Signed-off-by: Marian Balakowicz <m8@semihalf.com>