GPT: read partition table from device into a data structure

Make the partition table available for modification by reading it from
the user-specified device into a linked list.   Provide an accessor
function for command-line testing.

Signed-off-by: Alison Chaiken <alison@peloton-tech.com>
[trini: Make this depend on CMD_GPT_RENAME, as it is the user of this
code]
Signed-off-by: Tom Rini <trini@konsulko.com>