imagebuilder: cleanup package inclusion

imagebuilder: cleanup package inclusion

Previously three different ways to include packages in an ImageBuilder
existed:

* buildbot: include libc, kernel (and base-files) in $(IB_LDIR)
* not buildbot, standalone: include all packages in ./packages/
* not buildbot, not standalone: include libc, kernel (and base-files) in
  ./packages/

First of, the separation between *buildbot* and *not buildbot, not
standalone* is not required, we can just always copy packages to
./packages instead of ever using the special place $(IB_LDIR).
Doing so drops the need to handle the extra case and also allows to
clean up the OPKG package installation, which no longer requries the
`firstword` logic, things are now always at ./packages.

Signed-off-by: Paul Spooren <mail@aparcar.org>