wic: use kB for the partitions size

Use kB instead of MB for the partition size to get a better granularity.

This is needed on some SoC (i.mx, omap) where it is necessary to create
partitions as small as 64kB.

Keep the backward compatibility by assuming MB when no unit is provided.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Tested-by: Maciej Borzecki <maciej.borzecki@open-rnd.pl>
Acked-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Tested-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>