pkg-infra: correct the source name for host-only packages

Host-only package that don't define their <PKG>_SOURCE variable would
default to host-<pkg>-<version>.tar.gz.  It's more logical to remove
the host- prefix in this case.

This problem is most apparent with host-only packages downloaded from
version control, because they never define <PKG>_SOURCE.

Reported by Thomas Petazzoni and initial analysis by Luca Ceresoli.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>