scripts/feeds: observe -p flag for preferential feeds

lookup_target was trampling the $feed variable, resulting in the -p flag
no longer preferentially installing from the named feed.

Make sure to use a local variable for this instead.

Signed-off-by: Karl Palsson <karlp@remake.is>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46120 3c298f89-4303-0410-b956-a3cf2f4a3e73