mgmt/MCUmgr/grp/img: Improvement in image erase logic

The erase logic has been switched to using img_mgmt_get_opposite_slot
and the img_mgmt_get_other_slot has been removed.
The commit adds CONFIG_MCUMGR_GRP_IMG_ALLOW_ERASE_PENDING Kconfig
options, default set to n, that allows to make pending slot
erasable. The option only allows erase on pending slot that
is not revert slot.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>