[package] base/block-extroot, base/block-mount: Modified preinit and block-extroot and

block-mount so that use of block-mount and block-extroot do not require that block-extroot,
block-mount, nor the kernel modules they depend, on are required to included in the image.
That is block-extroot and dependencies may now be installed as modules onto the jffs2 part of
a squashfs system and it will work.
In addition packages which are installed into the jffs2 of a squashfs system may now affect
preinit, so long as they do not require execution of commands that occur before the merging of
the jffs2 and built-in (squashfs) preinit scripts is done.

Thanks jow for the preinit merge stuff!

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@23110 3c298f89-4303-0410-b956-a3cf2f4a3e73