Allow setting GPT partition names as used by TF-A bl2 to identify the
FIP volume to load from eMMC and SD-card.
While at it, also allow setting 'required' attribute as it should be
used for volumes which are essential for the system to boot.
Also properly handle setting the LEGACY_BOOT flag on the partition
selected as 'active', as this is how it is specified in the spec.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>