Implement USB flash drive installer as additional helper script.

[ivandavidov]

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The USB flash drive installer should be separate helper script in the MLL source tree and should work like this:

• Detect the overlay type.
• Create FAT partition and use it as EFI system partition (ESP). Use systemd-boot to implement proper UEFI boot manager structure.
• Use Extlinux as legacy BIOS boot manager. The kernel/initramfs files should be reused from the EFI system partition.
• Create Ext2 partition if necessary (see below).
• Depending on the overlay type the FAT/Ext2 partitions can be the following:
  • sparse - create FAT partition which uses the whole USB flash drive. Use the FAT partition to store the sparse image file there. No need to create Ext2 partition.
  • folder - create FAT partition which is just big enough to hold the boot files. Use all the left space for Ext2 partition and place the overlay structure there.
  • none - same as sparse but no need to deal with the overlay structure.
• The script is not supposed to work from the MLL runtime.

[ivandavidov]

For now the USB flash drive installation can be performed with dd. See #130 .