GPT: provide commands to selectively rename partitions

This patch provides support in u-boot for renaming GPT
partitions.  The renaming is accomplished via new 'gpt swap'
and 'gpt rename' commands.

The 'swap' mode returns an error if no matching partition names
are found, or if the number of partitions with one name does not equal
the number with the second name.   The 'rename' variant always
succeeds as long as a partition with the provided number exists.

Rewriting the partition table has the side-effect that all partitions
end up with "msftdata" flag set.  The reason is that partition type
PARTITION_BASIC_DATA_GUID is hard-coded in the gpt_fill_pte()
function.  This does not appear to cause any harm.

Signed-off-by: Alison Chaiken <alison@peloton-tech.com>