When USB network card drivers are used, the BIOS' legacy USB
capability is necessarily disabled since there is no way to share the
host controller between the BIOS and iPXE.  This currently results in
USB keyboards becoming non-functional in USB-enabled builds of iPXE.

Fix by adding basic support for USB keyboards, enabled by default in
iPXE builds which include USB support.

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