fix(chromeos-postinst): Copy pvgrub config from root to boot.

Previously the postinst tool shuffled menu.lst around on the boot
partition which doesn't allow the root filesystem a chance to provide a
new copy like it can for syslinux.
fix(chromeos-postinst): Do not copy over the entire syslinux directory.

There is already code to copy the correct root config and we shouldn't
go clobbering the alternate root or boot_kernel config.
cleanup(chromeos-postinst): Delete some dead code, cleanup var names.

Removing the in-place editing of syslinux configs left some remains
behind. Clean things up so the code looks consistent.
cleanup(chromeos-postinst): Remove support for alternate BIOS types.

Even when we run on a BIOS that needs no bootloader or special handling
we should still do the same updates in the boot partition. Otherwise
doing things like switching a host between UEFI and legacy mode may lead
to some unexpected results. With this the cros_legacy flag also dies.

Nothing really changes for us, this was pretty much all dead or broken
code.
fix(unittests): Fix tests that assumed old chromeos partition layout.
cleanup(chromeos-postinst): Remove references to old kernel partition.
fix(chromeos-postinst): Fix syslinux path on root partition.

Dropped /boot from the path by accident in 7012a1e4
Merge pull request #7 from marineam/bootloaders

Bootloaders