bootcount_ext: Add flag to enable/disable bootcount

After a successful upgrade, multiple problem during boot sequence may
trigger the altbootcmd process.
This patch adds a version and an upgrade_available entries to the
bootcount file to enable/disable the bootcount check.
When failing to read the bootcount file it will consider that bootcount is
enabled, acting as previously, and update the file accordingly.

The bootcount file is only saved when `upgrade_available` is true, this
allows to save writes to the filesystem.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>