Boot environment variables changes

[Michael Vogt]

Hi,

as part of our work to create device images, we recently changed and
simplified the boot environment variables that are used by the gadget
snap. If you work on creating gadget snaps, there are some small
tweaks that you need to do:

The "snappy_{os,kernel}" boot variable names got changed to
"snap_{core,kernel}".

The boot protocol changed from snappy_mode={regular,try} and
snappy_trival_boot={0,1} to snap_mode={"",try,trying}.

An example for the pi2/uboot implementation with the new logic can be
found here:
http://bazaar.launchpad.net/~snappy-dev/snappy-hub/snappy-systems/view/head:/pi2/boot-assets/uboot.env.in#L48

For most people this change is totally transparent, this only affects
you if you actually work on the bootloader configuration (grub.cfg or
uboot.env) of gadget snaps.

Cheers,
 Michael