lib/repo: Cleanup current boot's staging dir min-free-space-* checks are hit

lib/repo: Cleanup current boot's staging dir min-free-space-* checks are hit

min-free-space-* act as a gating condition whether to we want hold onto caches in
repo/tmp. If it is found that the free-disk space is going below this threshold,
we flag it as an error and cleanup current boot's staging directory.

Closes: #1602
Approved by: jlebon