[romprefix] Do not fall back to hooking INT19 by default

Several BIOSes (including most IBM BIOSes and many virtual machine
BIOSes) do not provide detectable PnP support, but will use the BEV
entry point for a PnP option ROM.  On these semi-PnP BIOSes, iPXE will
respond to the absence of detectable PnP support by hooking INT19,
which disrupts the boot order.

BIOSes that genuinely require hooking INT19 seem to be very rare
nowadays.  It may therefore be preferable to assume that the absence
of detectable PnP support indicates a semi-PnP BIOS rather than a
non-PnP BIOS.

Change the default behaviour so that INT19 will never be hooked unless
the compile-time option NONPNP_HOOK_INT19 is enabled.  Leave the
redundant PnP detection routine in-place to allow for debugging via
the ROM banner line.

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

src/arch/i386/prefix/romprefix.S

 	call	print_message
 	jmp	pnp_done
 no_pnp:	/* Not PnP-compliant - hook INT 19 */
+#ifdef NONPNP_HOOK_INT19
 	movw	$init_message_int19, %si
 	xorw	%di, %di
 	call	print_message
 	pushw	%gs /* %gs contains runtime %cs */
 	pushw	$int19_entry
 	popl	%es:( 0x19 * 4 )
+#endif /* NONPNP_HOOK_INT19 */
 pnp_done:
 
 	/* Check for PMM */

src/config/general.h

 #undef	PXE_CMD			/* PXE commands */
 #undef	REBOOT_CMD		/* Reboot command */
 
+/*
+ * ROM-specific options
+ *
+ */
+#undef	NONPNP_HOOK_INT19	/* Hook INT19 on non-PnP BIOSes */
+
 /*
  * Error message tables to include
  *