Add support for MTDPART_OFS_RETAIN: such partitions start at the current
offset, take as much space as possible, but rain part->size bytes after
the end of the partitions for other parts. Primarily this is intended
for ts72xx arm platforms cleanup.

Artem: tweaked the patch a bit
Signed-off-by: NDmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>