Petitboot v1.2.2 released

[Samuel Mendoza-Jonas]

Petitboot v1.2.2 has been tagged, with the following fixes over v1.2.1:

- ui/ncurses: Call widget process_key handlers first
	Fixes an issue where the left/right keys weren't properly
	recognised when a textbox field was focussed.

- Add 'sync' user event
	Allows the use of 'pb-event sync at device' to allow changes to
	snapshots to be manually synced back to disk.

- Add a much-requested README file

- Fixes two issues related to updating chosen/linux,stdout. In
particular fixes an issue with (at least) the RHEL 7.2 installer.