Using a single host package staging dir (and build dir) significantly speeds up
builds when multiple targets are built in succession, especially for large host
packages like NodeJS.

$(STAGING_DIR)/host is kept in addition to $(STAGING_DIR_HOSTPKG) in most
places; it is still used as destination for host files in Build/InstallDev.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>