part: detect EFI system partition

Up to now for MBR and GPT partitions the info field 'bootable' was set to 1
if either the partition was an EFI system partition or the bootable flag
was set.

Turn info field 'bootable' into a bit mask with separate bits for bootable
and EFI system partition.

This will allow us to identify the EFI system partition in the UEFI
sub-system.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>