[romprefix] Allow autoboot device filter to be disabled

Our current behaviour when booting as a ROM is to autoboot only from
devices which are attached via the PCI bus:dev.fn address passed to
the ROM's initialisation vector.

Add a build configuration option AUTOBOOT_ROM_FILTER (enabled by
default) to control this behaviour.  This allows for ROMs to be built
which will attempt to boot from any detected device, even if not
attached via the original PCI bus:dev.fn address.  (This is
particularly useful when building combined EHCI/xHCI ROMs for USB
network boot, since the BIOS may request a boot via the EHCI
controller but the xHCI driver will reroute the root hub ports to the
xHCI controller.)

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

src/arch/i386/prefix/romprefix.S

@@ -775,7 +775,9 @@ exec:	/* Set %ds = %cs */
 
 	/* Store PCI bus:dev.fn, if applicable */
 .ifeqs	BUSTYPE, "PCIR"
+#ifdef AUTOBOOT_ROM_FILTER
 	movw	%ax, autoboot_busdevfn
+#endif /* AUTOBOOT_ROM_FILTER */
 .endif
 
 	/* Call main() */

src/config/general.h

@@ -140,6 +140,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 #undef	NONPNP_HOOK_INT19	/* Hook INT19 on non-PnP BIOSes */
+#define	AUTOBOOT_ROM_FILTER	/* Autoboot only devices matching our ROM */
 
 /*
  * Error message tables to include