Always create /vendor/etc/{passwd,group}

Previously we only created these if the device set
TARGET_FS_CONFIG_GEN, however there are now other targets that want to
depend on these.  Instead of having those targets conditionally depend
on them, we always create them, defaulting to blank contents (by
reading /dev/null for TARGET_FS_CONFIG_GEN).

Test: builds succeed
Change-Id: Ie95286f5a800d891022eb66cd6fefcc967000c2e