[build] Create util/genfsimg for building filesystem-based images

[build] Create util/genfsimg for building filesystem-based images

Generalise util/geniso, util/gensdsk, and util/genefidsk to create a
single script util/genfsimg that can be used to build either FAT
filesystem images or ISO images.

Extend the functionality to allow for building multi-architecture UEFI
bootable ISO images and combined BIOS+UEFI images.

For example:

  ./util/genfsimg -o combined.iso \
      bin-x86_64-efi/ipxe.efi \
      bin-arm64-efi/ipxe.efi \
      bin/ipxe.lkrn

would generate a hybrid image that could be used as a CDROM (or hard
disk or USB key) on legacy BIOS, x86_64 UEFI, or ARM64 UEFI.

Signed-off-by: Michael Brown <mcb30@ipxe.org>