mkimg: Add -m and -C argument
-c is still used to specify a minimum capacity for the disk.
Add -m which specify a maximum capacity, if the size of the given
partition exceed it, mkimg will fail.
Add -C for specifying a fixed capacity size, this is equivalent of
specifying -c and -m with the same value
Add support for percent in partition size
If a maximum capacity is given (either by -m or -C) a partition size
can be given in percentage of the whole disk capacity (minus the metadata