mkimg: Add -C argument to specify maximum capacity

Add a -C option to specify a maximum capacity for the final image file.
It is useful to control the size of the generated image for sdcard or
when we will add dynamic size partition.

Add --capacity which is a shorthand to define min and max capacity at
the same time.

Reviewed by: bapt, marcel, wblock (manpages)
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D10509