no functional difference but make it clearer what the intention is

- removed serial from cmdline. rpi3 has a limited serial interface
  (used for bluetooth now) and when enabled (enable_uart=1) it
  will lock the cpu speed to 250Mhz.

ext4 module can handle it.

standard iso is now vanilla kernel so it does not make any sense to have
a separata vanilla iso.

Also update xen to use vanilla kernel instead of hardened

ref #7181