fix(base_image_util): Remove dead code, leave rootfs writable.

The verification flag was being passed through to the bootloader
template script but no longer had any effect.
Force the base image to always remain writable, its only purpose is to
be modified in a later build step anyway.
fix(build_image): Migrate to new disk_util commands
fix(cros_make_image_bootable): Remove dead command line flags.

Lots of things are either unused or meaningless. A particularly creative
one is the fact that there are command line flags for mount point
locations that are then overwritten.
fix(disk_util): Install syslinux as part of filesystem formatting.

This means scripts later don't need to deal with the ESP block device
directly, they can just copy the config files into place.
fix(create_legacy_bootloader_templates): Setup ESP partition.

Replace "update_bootloaders" script.
fix(cros_make_image_bootable): Switch to disk_util mount/umount
add(prod_image_util): Rename what remains of cros_make_image_bootable

cros_make_image_bootable now only is relevant for prod images, so move
the remaining code to prod_image_util in a similar scheme that base and
dev images use.