[pcbios] Don't use "lret $2" to return from an interrupt

Using "lret $2" to return from an interrupt causes interrupts to be
disabled in the calling program, since the INT instruction will have
disabled interrupts.  Instead, patch CF on the stack and use iret to
return.

Interestingly, the original PC BIOS had this bug in at least one
place.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Michael Brown <mcb30@etherboot.org>

src/arch/i386/interface/pxe/pxe_entry.S

@@ -199,9 +199,12 @@ pxe_int_1a:
 	shll	$4, %edx
 	addl	$pxenv, %edx
 	movw	$0x564e, %ax
+	pushw	%bp
+	movw	%sp, %bp
+	andb	$~0x01, 8(%bp)	/* Clear CF on return */
+	popw	%bp
 	popfw
-	clc
-	lret	$2
+	iret
 1:	/* INT 1A,other - pass through */
 	popfw
 	ljmp	*%cs:pxe_int_1a_vector