cleanup(common): Remove rarely used pv_cat_cmd

Its single use is in build_common and even then having a little progress
bar for copying images isn't that interesting, they just get lost in the
noise of the emerge output. Keep it simple, use cp.
fix(build_image): Fix building dev and prod without base

Ever since adding prod it hasn't been possible to build just dev and
prod without explicitly building base. Base is always built but usually
there is no point to keeping it around. Add some logic to make dev not
conflict with prod and make sure base is deleted and not uploaded if it
wasn't explicitly requested.
fix(build_library): Use sparse files for disk images, no useless sudo

Enable sparse files for all dd and cp commands and replace some dd
commands that are really better off being truncate commands.

While in the neighborhood there were a number of useless sudo commands
for things that just happen to be in sbin. Call them directly instead.
fix(catalyst): Cleanup temporary files.

Catalyst doesn't clean up the temporary root directories even though we
don't have seedcache enabled. So lets wipe that mess up.
Merge pull request #65 from marineam/misc

Misc build updates