cmd: Add MBR partition layout control utility

Add a 'mbr' command to let users create or verify MBR partition layout
based on the provided text description. The partition layout is
alternatively read from the 'mbr_parts' environment variable. This can be
used in scripts to help system image flashing tools to ensure proper
partition layout.

The syntax of the text description of the partition list is similar to
the one used by the 'gpt' command. Supported parameters are: name
(currently ignored), start (partition start offset in bytes), size (in
bytes or '-' to expand it to the whole free area), bootable (boolean
flag) and id (MBR partition type). If one wants to create more than 4
partitions, an 'Extended' primary partition (with 0x05 ID) has to be
explicitely provided as a one of the first 4 entries.

Here is an example how to create a 6 partitions (3 on the 'extended
volume'), some of the predefined sizes:

> setenv mbr_parts 'name=boot,start=4M,size=128M,bootable,id=0x0e;
  name=rootfs,size=3072M,id=0x83;
  name=system-data,size=512M,id=0x83;
  name=[ext],size=-,id=0x05;
  name=user,size=-,id=0x83;
  name=modules,size=100M,id=0x83;
  name=ramdisk,size=8M,id=0x83'
> mbr write mmc 0

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>