boot: bootutil: avoid full erase on BOOT_MAGIC_BAD in boot_set_next

Previously, boot_set_next() would perform a full slot erase when
BOOT_MAGIC_BAD was detected. This is no longer done, as the
decision on how to handle a corrupted image should be left to user code.

Signed-off-by: Ofir Shemesh <ofirshemesh777@gmail.com>