[romprefix] Display only one "Ctrl-B" prompt per PCI device during POST

If a multifunction PCI device exposes an iPXE ROM via each function,
then each function will display a "Press Ctrl-B to configure iPXE"
prompt, and delay for two seconds.  Since a single instance of iPXE
can drive all functions on the multifunction device, this simply adds
unnecessary delay to the boot process.

Fix by inhibiting the "Press Ctrl-B" prompt for all except the first
function on a PCI device.

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

src/arch/i386/prefix/romprefix.S

 	( PMM_HANDLE_BASE | 0x00001000 )
 #define PMM_HANDLE_BASE_DECOMPRESS_TO \
 	( PMM_HANDLE_BASE | 0x00002000 )
+#define PCI_FUNC_MASK 0x07
 
 /* ROM banner timeout.  Based on the configurable BANNER_TIMEOUT in
  * config.h, but converted to a number of (18Hz) timer ticks, and
 	xorw	%di, %di
 	cs rep	movsb
 
+	/* Skip prompt if this is not the first PCI function */
+	testb	$PCI_FUNC_MASK, init_pci_busdevfn
+	jnz	no_shell
 	/* Prompt for POST-time shell */
 	movw	$init_message_prompt, %si
 	xorw	%di, %di
 	movw	$init_message_done, %si
 	call	print_message
 	popf
-	jnz	2f
+	jnz	no_shell
 	/* Ctrl-B was pressed: invoke iPXE.  The keypress will be
 	 * picked up by the initial shell prompt, and we will drop
 	 * into a shell.
 	xorl	%ebp, %ebp	/* Inhibit use of INT 15,e820 and INT 15,e801 */
 	pushw	%cs
 	call	exec
-2:
+no_shell:
+	movb	$( '\n' ), %al
+	xorw	%di, %di
+	call	print_character
+
 	/* Restore registers */
 	popw	%gs
 	popw	%fs
  *
  */
 init_pci_busdevfn:
-	.word	0xffff
+	.word	0
 	.size	init_pci_busdevfn, . - init_pci_busdevfn
 
 /* Image source area