Otherwise the initrd file will not boot the kernel, instead returning a Kernel Panic and "initramfs unpacking failed: junk in compressed archive". lsinitcpio -a was helpful in spotting this issue: ==> ERROR: Newer lz4 stream format detected! This may not boot!

lz4 -l is necessary for Linux kernel boot compatibility, a separate branch and patch has already been pull request for that change in code. This also updates the documentation.

Sync dev/master branches

mkrescue on a source computer with both USB 2.0 and 3.0 port might run with the xhci module only.
When the (USB) boot device is used to start a USB 2.0 computer that requires the ehci module, both the ehci_hcd and ehci_pci modules, to boot with keyboard support. Tested with Arch Linux 4.9.11-1 on an Intel DE3815TYKHE (USB2+3 where lsmod only show the xhci module), then ReaR will not have keyboard support when booted on a machine with USB 2.0 only. After adding the 4 (u|e|x|o)hci_pci modules, the USB keyboard and mouse do function again with the USB 2.0 only mainboard.

Create initrd.lz4 with lz4 -l compression
'-l' = legacy format is preferred because
otherwise it may fail to get uncompressed,
see #1218

Document initrd.lz4 with lz4 -l compression
and comparison with other compression methods,
see #1218

Add USB (u|e|x|o)hci_pci modules for broader hardware support.
If one of them is missing USB keyboard may not work on
replacement hardware with different USB support, namely
USB3 (where usually only xhci works) versus USB2.
USB keyboard function on any replacement hardware
is a must nowadays so that ReaR should by default
include all needed USB modules in its recovery system.