remap: add remap_type field and use it

Maintain backwards compatibility for old images, but don't set the REMAP_GHOST
bit going forward, only use the remap_type field.

v2: * preserve remap_id in GHOST_REMAP case
    * protobuf field is remap_type enum not u32

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>