remove the notion of "root path"; support mixed flash types

Remove the wacky notion of "roots" and "root paths" (those things that
look like "FOO:some/path" instead of just "/foo/some/path").  Let each
device specify its own table of available partitions and how to mount
them (needed for devices that use both MTD/yaffs2 and EMMC/ext4
partitions).

(Cherrypicked from gingerbread w/slight edits.)

Change-Id: I2479ce76b13e73f1d12035c89386c3a82b3edf51