[console] Allow consoles to update the recorded console size

[console] Allow consoles to update the recorded console size

Provide a mechanism for consoles to update the recorded console width
and height, and use this width and height to provide the curses COLS
and LINES variables.

We choose not to use ANSI escape sequences to obtain the width and
height, for two reasons:

- iPXE's model is that all output is sent to all consoles; we could
  therefore end up with multiple consoles reporting conflicting widths
  and heights

- when a serial console is in use, we probably don't want to resize
  the output shown on the BIOS console to match the size of the serial
  console, since it's likely that the serial console is in use only
  for debugging.

Signed-off-by: Michael Brown <mcb30@ipxe.org>