gpt: add optional parameter type in gpt command

code under flag CONFIG_PARTITION_TYPE_GUID
add parameter "type" to select partition type guid

example of use with gpt command :

  partitions = uuid_disk=${uuid_gpt_disk}; \
      name=boot,size=0x6bc00,uuid=${uuid_gpt_boot}; \
      name=root,size=0x7538ba00,uuid=${uuid_gpt_root}, \
         type=0fc63daf-8483-4772-8e79-3d69d8477de4;

  gpt write mmc 0 $partitions

Signed-off-by: Patrick Delaunay <patrick.delaunay73@gmail.com>