STAGING_DIR may be provided from command line. We currently hardcoded
STAGING_DIR_HOST and STAGING_DIR_HOSTPKG to the default location but we
currently have some relocatable patch that derive the path from
STAGING_DIR.

Fix this and correctly derive STAGING_DIR_HOST and STAGING_DIR_HOSTPKG
from STAGING_DIR.

The intention is to fix inconsistency from the relocatable patch and the
use of STAGING_DIR_HOST that is always hardcoded.
This with a wrong configuration may end up in broken state with some
host tools expecing a PATH from STAGING_DIR and others using library
from the default staging_dir/host path.

To save downstream project the original implementation is saved while
fixing the inconsistency between patch and .mk.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>