base-files: use less entropy when generating ULAs

git-svn-id: svn://svn.openwrt.org/openwrt@36865 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: backport r36865

git-svn-id: svn://svn.openwrt.org/openwrt@36866 3c298f89-4303-0410-b956-a3cf2f4a3e73