gpt: Support for new "gpt" command

New command - "gpt" is supported. It restores the GPT partition table.
It looks into the given environment variable for partitions definition.
It can be enabled at target configuration file with CONFIG_CMD_GPT.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Piotr Wilczek <p.wilczek@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>