package: allow to override the make install target for host packages

In order to convert gcc to the package infrastructure, we need a way
of overriding the name of the make target used for the installation of
host packages, just like we have for target packages.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>