The bootcount will soon change to be store on the ext filesystem
in /boot/failures. The formar is two bytes { 0xbc, <bootcount> }.
To handle this new format, the update manager needs an update.
The bootcount is no longer saved, only u-boot will have to write it.
The upgrade_available variable is not available, so the file is written
at each boot.
The bootlimit is hardcoded in the update-manager and must stay in sync
with u-boot.

Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>