package/pkg-generic.mk: don't set INSTALL_{TARGET, STAGING, IMAGES} for host packages

package/pkg-generic.mk: don't set INSTALL_{TARGET, STAGING, IMAGES} for host packages

By their very nature, host packages have no target, staging, or images
install steps; they have a single install step, that is always
performed.

As such, setting the corresponding _INSTALL_{TARGET,STAGING,IMAGES}
variables does not make sense for host packages.

However, people (and scripts) may get confused when they process the
output of printvars, e.g.:

    $ make printvars VARS=HOST_LIBTOOL_INSTALL_TARGET
    HOST_LIBTOOL_INSTALL_TARGET=YES

Only set those variables for target packages. There is no
corresponding variable for host packages, as they are always installed
(and only once).

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: eeppeliteloop@gmail.com
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>