[oe] Cleaning tmp folder

[Frans Meulenbroeks]

A crude temp solution could be to just remove the stamps of all
non-native, non-cross packages.

Also if you only want to reclaim space I guess it is ok to remove the
work subdir (or at least in my case (beagleboard) the work/arm* dir)
If this is not right I guess someone will correct me :-)

Btw: it would also be nice to have a command to purge the tmp dir, so
if X build for r0 is there and X build for r1 is there the r0 version
is nuked. Never got to writing something for that though :-)

Frans.