wic: allow creation of partitions not in table

wic: allow creation of partitions not in table

For some architectures it is necessary to reserve space on disk without
it being present in the partition table.

For example, u-boot on i.mx is placed at an offset of 1kB on the sdcard.
While it would be possible to create a partition at that offset and
place u-boot there, it would then be necessary to update the default
u-boot environment to use partition 2 on the mmc instead of partition 1.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>