Enhanced do_mounts_dm to handle multiple devices

Enhanced do_mounts_dm to handle multiple devices

Handling mutliple device-mappers is preperation for adding
the boot cache.

The change to optionally accept <num> fields is a temporary
measure until all scripts supply the <num> fields, then the
code will be changed to make it required.

BUG=chromium-os:25441
TEST=booted stumpy and ran code with a number of tests in a mock
	test environment.

Change-Id: I10b9c21d2b76763872a91053db94a8354429f33d
Signed-off-by: Paul Taysom <taysom@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/27311
Reviewed-by: Kees Cook <keescook@chromium.org>