[romprefix] Change from opt-in to opt-out when booting via INT19

On non-BBS systems, we have to hook INT 19 in order to be able to boot
from the gPXE ROM at all.  However, doing this unconditionally will
prevent the user from booting via any other devices.

Previously, the INT 19 entry point would prompt the user to press B in
order to boot from gPXE, which makes it impossible to perform an
unattended network boot.  We now prompt the user to press N to skip
booting from gPXE, which allows for unattended operation.

This should be a better match for most real-world scenarios.  Most
modern systems support BBS and so are unaffected by this change.  Very
old (non-BBS) systems tend not to have PXE ROMs by default anyway; if
the user has added a gPXE ROM then they probably do want to boot from
the network.  Newer non-BBS systems are essentially limited to IBM
servers, which will recapture the INT 19 vector anyway and implement
their own boot-ordering selection mechanism.

src/arch/i386/prefix/romprefix.S

 	call	print_message
 	movw	$int19_message_dots, %si
 	call	print_message
-	movw	$0xdf42, %bx
+	movw	$0xdf4e, %bx
 	call	wait_for_key
 	pushf
 	xorw	%di, %di
 	movw	$int19_message_done, %si
 	call	print_message
 	popf
-	jnz	1f
+	jz	1f
 	/* Leave keypress in buffer and start gPXE.  The keypress will
 	 * cause the usual initial Ctrl-B prompt to be skipped.
 	 */
 	.size	orig_int19, . - orig_int19
 
 int19_message_prompt:
-	.asciz	"Press B to boot from "
+	.asciz	"Press N to skip booting from "
 	.size	int19_message_prompt, . - int19_message_prompt
 int19_message_dots:
 	.asciz	"..."