[hci] Draw all widgets on the standard screen

[hci] Draw all widgets on the standard screen

The curses concept of a window has been supported but never actively
used in iPXE since the mucurses library was first implemented in 2006.

Simplify the code by removing the ability to place a widget set in a
specified window, and instead use the standard screen for all drawing
operations.

This simplification allows the widget set parameter to be omitted for
the draw_widget() and edit_widget() operations, since the only reason
for its inclusion was to provide access to the specified window.

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