This commit updates all backends that write to /mnt/boot to do it
through a new `lib/host-utils` module. Writes are now done using write +
sync as rename is not an atomic operation in vfat.
The change also applies for writes through the `/v1/host-config`
endpoint.
Finally this change includes some improvements on tests.
Change-type: patch
We add a bunch of additional unit tests, and also a coverage report using istanbul.
The tests are not meant to cover everything, but they're a first attempt at having *some* unit testing
on the supervisor. There's much to improve but hopefully it helps catch obvious errors.
Change-Type: patch
Signed-off-by: Pablo Carranza Velez <pablo@resin.io>
