cmd:gpt: randomly generate each partition uuid if undefined

Changes:
- randomly generate partition uuid if any is undefined and CONFIG_RAND_UUID
  is defined
- print debug info about set/unset/generated uuid
- update doc/README.gpt

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Piotr Wilczek <p.wilczek@samsung.com>
Cc: Tom Rini <trini@ti.com>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>