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
'mz' can be safely replaced with fs.promises
and util.promisify for faster native methods.
'mkdirp' after Node v8 uses native fs.mkdir, thus
is redundant. 'body-parser' is deprecated and
contained within express v4.x.
Closes: #1567
Change-type: patch
Signed-off-by: Christina Wang <christina@balena.io>