[all-snap] only set snappy_{os,kernel} to snappy_good_{os,kernel} if the later is non-empty

[Gustavo Niemeyer]

(from https://github.com/ubuntu-core/snappy/issues/5)

mvo commented on Oct 22, 2015:

Right now snappy_{os,kernel} will be set on trial_boot=1 to snappy_good_{os,kernel}. However on first boot when snappy_good_{os,kernel} is empty (and trial_boot is set to 1 by grub) there is a race that if someone reboots before book-ok is run the snappy_{os,kernel} is unset. So the grub/uboot cfg needs to check if snappy_good_{os,kernel} is empty or not.

[Michael Vogt (mvo)]

This is fixed in ubuntu-device-flash.