[romprefix]: Allow to enforce PnP BIOSes to be detected as non-PnP

Add a define NONPNP_FORCE that controls building of the PnP detection
routine. Together with the existing NONPNP_HOOK_INT19 hook this enables
booting iPXE on PnP BIOSes that can not be persuaded to boot iPXE by
BIOS menu configuration alone (e.g. my abit BX6 2.0).

Signed-off-by: Stefan Tauner <stefan.tauner@gmx.at>