cmd: gpt: Add command to swap partition order

Adds a command called "gpt transpose" which will swap the order two
partition table entries in the GPT partition table (but leaves them
pointing to the same locations on disk).

This can be useful for swapping bootloaders in systems that use an A/B
partitioning scheme where the bootrom is hard coded to look for the
bootloader in a specific index in the GPT partition table.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>