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
(GPE).

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.

Current limitations:

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. It also
ignores all GPE attributes.