CC: base-files: use the correct path in case of modified preinit options

CC: base-files: use the correct path in case of modified preinit options
If the user sets any preinit options in .config, the wrong path may get
applied due to wrong default value in image-config.in and due to Makefile
writing also the unchanged options into 00_preinit.conf

Modify the default path in image-config.in to match the current default
path set by r47080. Also modify the fall-back default in Makefile.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
Backport of r47590

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@48238 3c298f89-4303-0410-b956-a3cf2f4a3e73