pkg-infra: improve reliability of foo-install

Running foo-uninstall has a high likelyhood of failing, because we
remove a lot of directories from the target.  To improve the reliability,
remove the stamp files before calling the uninstall commands, and add
the -k option to the sub-make.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>