Custom partition table creation fixes and improvements

[achilleas-k]

• We use ef02 for the BIOS boot partition on MBR/DOS. This is incorrect. The BIOS boot partition on MBR is actually not required. We may keep it for consistency (probably not the right thing to do) and, if we do, we should change the type.
  • Type changed to 00 empty, but it's still partitioned space.
    • Add support for selecting a partition table type #1085
• Change ef00 for ESP on DOS to ef.
  • Add support for selecting a partition table type #1085
• Produce errors when more than 4 partitions are created on MBR PTs.
  • Add support for selecting a partition table type #1085
• When a fs_type is missing, the error message is not helpful: invalid partitioning customizations: unknown or invalid filesystem type for mountpoint "/var" We should somehow mention that the field is called fs_type.
  • blueprint: clarify filesystem type customization name #1097
• Use partition type ID constants everywhere (Add support for selecting a partition table type #1085 (comment)).
  • disk: rename all DOS type ID consts to match GUIDs #1099

[achilleas-k]

From https://www.gnu.org/software/grub/manual/grub/html_node/BIOS-installation.html#BIOS-installation

MBR gap has few technical problems. There is no way to reserve space in the embedding area with complete safety, and some proprietary software is known to use it to make it difficult for users to work around licensing restrictions. GRUB works around it by detecting sectors by other software and avoiding them and protecting its own sectors using Reed-Solomon encoding.

GRUB team recommends having MBR gap of at least 1000 KiB.

We currently make a 1 MiB bios boot partition for dos/mbr disks the same way we do for gpt. This limits the usefulness of dos/mbr disks which is already limited to 4 partitions, essentially making it only 3 usable partitions in cases where BIOS booting is needed. We should instead use a gap as described in the docs.