[pxe] Initialize EDX on PXE NBP entry and INT 1Ah

The PXE 1.x spec specifies that on NBP entry or on return from INT
1Ah AX=5650h, EDX shall point to the physical address of the PXENV+
structure.  The PXE 2.x spec drops this requirement, simply stating
that EDX is clobbered.  Given the principle "be conservative in what
you send, liberal in what you accept", however, we should implement
this anyway.

src/arch/i386/interface/pxe/pxe_call.c

  * @ret rc		Return status code
  */
 int pxe_start_nbp ( void ) {
-	int discard_b, discard_c;
+	int discard_b, discard_c, discard_d;
 	uint16_t rc;
 
 	/* Far call to PXE NBP */
 					   "lcall $0, $0x7c00\n\t"
 					   "addw $4, %%sp\n\t" )
 			       : "=a" ( rc ), "=b" ( discard_b ),
-			         "=c" ( discard_c )
-			       :  "a" ( __from_text16 ( &ppxe ) ),
-			          "b" ( __from_text16 ( &pxenv ) ),
-			          "c" ( rm_cs )
-			       : "edx", "esi", "edi", "ebp", "memory" );
+				 "=c" ( discard_c ), "=d" ( discard_d )
+			       : "a" ( __from_text16 ( &ppxe ) ),
+			         "b" ( __from_text16 ( &pxenv ) ),
+			         "c" ( rm_cs ),
+			         "d" ( virt_to_phys ( &pxenv ) )
+			       : "esi", "edi", "ebp", "memory" );
 
 	return rc;
 }

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

  * Returns:
  *   %ax : 0x564e
  *   %es:bx : Far pointer to the PXENV+ structure
+ *   %edx : Physical address of the PXENV+ structure
  *   CF cleared
  * Corrupts:
  *   none
 	cmpw	$0x5650, %ax
 	jne	1f
 	/* INT 1A,5650 - PXE installation check */
-	pushw	%cs
-	popw	%es
+	xorl	%edx, %edx
+	movw	%cs, %dx
+	movw	%dx, %es
 	movw	$pxenv, %bx
+	shll	$4, %edx
+	addl	$pxenv, %edx
 	movw	$0x564e, %ax
 	popfw
 	clc