gpt: command: Extend gpt command to support GPT table verification

This commit adds support for "gpt verify" command, which verifies
correctness of on-board stored GPT partition table.
As the optional parameter one can provide '$partitons' environment variable
to check if partition data (size, offset, name) is correct.

This command should be regarded as complementary one to "gpt restore".

Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Przemyslaw Marczak <p.marczak@samsung.com>