package/pkg-generic.mk: make HOST_<pkg>_DL_OPTS inherit from <pkg>_DL_OPTS

package/pkg-generic.mk: make HOST_<pkg>_DL_OPTS inherit from <pkg>_DL_OPTS

Just like _SITE, _SOURCE, _SITE_METHOD, it is very likely that if
<pkg>_DL_OPTS is defined, the same value should be used for
HOST_<pkg>_DL_OPTS, so let's have the same inheritance logic than the
one we have for other variables.

Fixes:

  https://bugs.busybox.net/show_bug.cgi?id=12321

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>