feat(partition): cleanup partition and gpt headers

The EFI_NAMELEN macro has been moved to efi.h header. Get the macro
from efi.h. Use the struct efi_guid structure for declaring GUID
members in gpt.h

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: I1c3a2605b9f857b9cf2dcfdaed4dc9d0a2cbf0f0