gpt: part: Definition and declaration of GPT verification functions

gpt: part: Definition and declaration of GPT verification functions

This commit provides definition and declaration of GPT verification
functions - namely gpt_verify_headers() and gpt_verify_partitions().
The former is used to only check CRC32 of GPT's header and PTEs.
The latter examines each partition entry and compare attributes such as:
name, start offset and size with ones provided at '$partitions' env
variable.

Signed-off-by: Lukasz Majewski <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Przemyslaw Marczak <[email protected]>