ffspart: Rework command line size arguments

[shenki]

Currently the ffspart command line requires you to specify:

• block size (in hex?!)
• number of blocks

A friendlier UI would be to specify the:

• optionally, total output size in megabytes
• optionally, the block size

The description of the ffs gives the total size of the ffs image being crated, so the only reason a user should specify it is when they want a pnor that is larger than the partition data they have.

The block size is not required for creating a ffs layout. It is a good idea to create a layout where the partitions lie on eraseblock boundaries, and therefore are at least one earaseblock in size, but this is not required.

This could fix #202 too.