Fix erase of possible trailer leftover after reset

This fixes an issue where an image might be erased, but a trailer left
behind. It can happen if the image in the secondary slot did not pass
validation, in which case the whole slot is erased. If during the erase
operation, a reset occurs, parts of the slot might have been erased
while some have not. The concerning part is the trailer because it
might disable a new image from being loaded through mcumgr; so just get
rid of the trailer here, if the header is erased.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>