build: fix triggering opkg/host compilation

Commit 131db366 "build: remove separate /install step for host builds" dropped
the package/*/host/install targets in favor to performing the install steps
within the compile target instead.

Adjust package/Makefile accordingly in order to prevent a missing
staging_dir/host/bin/opkg when staging package archives into the rootfs.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>