fix(legacy/postinst): update for CoreOS partition layout

we need to put the menu.lst in place for pygrub and pvgrub on an upgrade
we need to use the right partition numbers

Change-Id: Ib899c6d727cfbfdd3cd5ce23280bb66b48ccee03
TODO: use a library like libblkid instead of hard coded numbers
hack(installer): don't depend on Kernel partition

delete all logic depending on the Kernel partition for now. This also
removes the verity logic.

Change-Id: I510b33808c3ded8a38847e0ed330da5745cc9d77