mtd: cmdlinepart: make the partitions rule more strict

Huang Shijie <shijie8@gmail.com> explains:

Assume we have a 1GiB(8Gib) NAND chip, and we set the partitions
in the command line like this:
    #gpmi-nand:100m(boot),100m(kernel),1g(rootfs)

In this case, the partition truncating occurs. The current code will
get the following result:

     ----------------------------------
        root@freescale ~$ cat /proc/mtd
        dev:    size   erasesize  name
        mtd0: 06400000 00040000 "boot"
        mtd1: 06400000 00040000 "kernel"
     ----------------------------------

It is obvious that we lost the truncated partition `rootfs` which should
be 824MiB in this case.

Also, forbid 0-sized partitions.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>