kernel - Update loader.conf documentation and add sio comconsole unit specifier

kernel - Update loader.conf documentation and add sio comconsole unit specifier

* Update the loader.conf documentation to describe how the boot loader and
  kernel can use a serial port and baud rate other than the default for
  comconsole.

  Also describe how to recompile the boot loader.

  Describe how IPMI can be tied into the boot loader and kernel via
  the built-in fake serial port in the BMC (often COM2).

* Add a kenv feature to allow the sioN.flags to be specified in loader.conf.
  This allows the comconsole to be moved to e.g. sio1 without having
  to recompile the kernel.