[pxe] Avoid drawing menu items on bottom row of screen

[pxe] Avoid drawing menu items on bottom row of screen

Many consoles will scroll immediately upon drawing a character in the
rightmost column of the bottom row of the display, in order to be able
to advance the cursor to the next character (even if the cursor is
disabled).

This causes PXE menus to display incorrectly.  Specifically, pressing
the down arrow key while already on the last menu item may cause the
whole screen to scroll and the line to be duplicated.

Fix by moving the PXE menu one row up from the bottom of the screen.

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