fs/iso9660: add support for hybrid image using Grub2 on BIOS and EFI

fs/iso9660: add support for hybrid image using Grub2 on BIOS and EFI

Add support for building an hybrid ISO9660 image compatible with legacy
and UEFI BIOS. Note that this is not about an (iso)hybrid image, which
can boot from both a CDROM or a USB stick, but really about an image
being bootable from the legay BIOS or EFI; the two are orthognal.

The option -eltorito-alt-boot need to be used in the xorriso command
to generate the hybrid image. That option is a separator, meaning the
previous boot entry is done, and the following boot options define a new
boot entry.

The -no-emul-boot defines the type of the current boot image; that's why
it has to now be repeated in each of the BIOS and EFI options.

Finally, for symetry and consistency between the BIOS and EFI options,
we move the BIOS image option first.

Note: the BIOS boot image options have to be provided before the EFI
ones, or the system won't boot; the underlying reason is not known...

Signed-off-by: Kory Maincent <[email protected]>
[[email protected]:
  - note about hybrid vs. (iso)hybrid
  - explain -eltorito-alt-boot
  - explain duplication of -no-emul-boot
  - rename the variables
  - note about the BIOS-EFI ordering
]
Signed-off-by: Yann E. MORIN <[email protected]>