fix(build_image): Remove noinitrd from default boot_args
cleanup(boot): remove lingering dm-verity bits, cleanup kernel args.

Remove unused dev/dm-0 vs dm-1 logic from verity and the associated
rootwait option it required (meaningless with our initrd). Move old
cros_legacy to common instead of using it in every command line option.
We should remove it entirely soon since it isn't useful for us. Remove
unneeded intel graphics modeset option.
fix(boot): Enable serial console when booting via syslinux.

Useful for qemu -nographic or any any other situation where serial is
easier to get at than VGA. It may be possible that in some setups ttyS0
isn't appropriate but we can figure out a way to customize kernel
options if/when that ever comes up.
Merge pull request #86 from marineam/bootargs

Bootargs