include/package.mk: remove old configured stamps before attempting configuration
Some packages, e.g. busybox, explicitly remove old .configured stamps
before attempting configuration, rather than after the actual configuration
step. This seems like a good idea, as there will be no stamp left if
configuration fails. Change generic rules to work like this, so package-
specific rules can be dropped.
Signed-off-by: Matthias Schiffer <[email protected]>