common: cmd_part: start and size sub-commands introduction

This introduces the part start and part size sub-commands. The purpose of these
is to store the start block and size of a partition in a variable, given the
device and partition number.

This allows reading raw data that fits a single partition more easily.
For instance, this could be used to figure out the start block and size of a
kernel partition when a partition table is present, given the partition number.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Acked-by: Stephen Warren <swarren@nvidia.com>
[trini: Change "%lx" to LBAF]
Signed-off-by: Tom Rini <trini@konsulko.com>