[romprefix] Allow ROM banner timeout to be configured independently

iPXE currently prints a "Press Ctrl-B" banner twice: once when the ROM
is first called for initialisation and again if we attempt to boot
from the ROM.  This slows boot, especially when the NIC is not the
primary boot device.  Tools such as libguestfs make use of QEMU VMs
for performing maintenance on disk images and may make use of NICs in
the VM for network support.  If iPXE introduces a static init-time
delay, that directly translates to increased runtime for the tools.

Fix by allowing the ROM banner timeout to be configured independently
of the main banner timeout.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>