lib/deploy: Port final bootconfig writing to new style

lib/deploy: Port final bootconfig writing to new style

The main blocker for doing this before was the `goto out` handling
for remounting `/boot`.  Handle that by factoring out the bits that
require it to a helper function, and do the C/GError equivalent of
"try/finally".

Not prep for anything right now, just decided to do this since I had the file
open.

Closes: #1515
Approved by: jlebon