[pxe] Add a dummy PXENV+ entry point at UNDI_CS:0000

Wyse Streaming Manager server (WLDRM13.BIN) assumes that the PXENV+
entry point is at UNDI_CS:0000; apparently, somebody at Wyse has
difficulty distinguishing between the words "may" and "must"...

Add a dummy entry point at UNDI_CS:0000, which just jumps to the
correct entry point.

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

  *   none
  ****************************************************************************
  */
+	/* Wyse Streaming Manager server (WLDRM13.BIN) assumes that
+	 * the PXENV+ entry point is at UNDI_CS:0000; apparently,
+	 * somebody at Wyse has difficulty distinguishing between the
+	 * words "may" and "must"...
+	 */
+	.section ".text16.null"
+	.code16
+pxenv_null_entry:
+	jmp	pxenv_entry
+
 	.section ".text16"
 	.code16
 pxenv_entry:

src/arch/i386/scripts/i386.lds

     . = _text16_link_addr;
     _text16 = .;
 
-    . += 1;			/* Prevent NULL being valid */
+    /* We need to allow code at the NULL address in .text16 */
 
     .text16 : AT ( _text16_load_offset + __text16 ) {
 	__text16 = .;
+	*(.text16.null)
+	. += 1;			/* Prevent NULL being valid */
 	*(.text16)
 	*(.text16.*)
 	_etext16_progbits = .;