Set snappy_trial_boot to "0" instead of unsetting it
he reason is that uboot is acting really funny when vars are unset.
I.e. "if "${snappy_trial_boot}" = "1"; then echo yes; fi" will echo yes
if snappy_trial_boot is unset on older u-boot systems (e.g. odroid).
Only changing uboot in 15.04 (not grub as we do in wily). by mvo approved by mvo,chipaca