part_blk: add GPT support

The GUID partition table (GPT) is primarily used by systems using
(U)EFI and is a replacement for the legacy MBR. For now, the current
implementation is able to address up to 128 GUID partition entries.
Since no endian conversion takes place it only works on LE platforms
and of all characters in the UTF-16 encoded name field of an entry
only the ones included in the ASCII encoding are printed.

To enable the GPT support in 'part_blk' it has to be configured
accrodingly:

! <start name="part_blk">
! [...]
!   <config use_gpt="yes">
! [...]
! </start>

If 'part_blk' is not able to find a valid GPT header it falls back
to using the MBR.