[settings] Add support for navigation keys in "config" user interface

Add support for page up, page down, home and end keys, matching the
navigation logic used in the menu user interface.

Originally-implemented-by: Marin Hannache <git@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[realtek] Allow extra space in RX buffers

Some hardware (observed with an onboard RTL8168) will erroneously
report a buffer overflow error if the received packet exactly fills
the receive buffer.

Fix by adding an extra four bytes of padding to each receive buffer.

Debugged-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>