classes/rootfs_deb: use more reliable check for package existence

It turns out "apt-cache showpkg" does return some information when a
package does not exist but another package recommends it, which can
occur for empty *-dev packages; so use "apt-cache policy" with a
different line count instead.

(From OE-Core rev: ac62761a9cacdfd0225d2db5a75584e6c85469a8)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/rootfs_deb.bbclass

@@ -111,7 +111,7 @@ list_package_recommends() {
 }
 
 rootfs_check_package_exists() {
-	if [ `apt-cache showpkg $1 | wc -l` -gt 2 ]; then
+	if [ `apt-cache policy $1 | wc -l` -gt 4 ]; then
 		echo $1
 	fi
 }