[librm] Use libflat to enable A20 line on each real-to-protected transition

Use the shared code in libflat to perform the A20 transitions
automatically on each transition from real to protected mode.  This
allows us to remove all explicit calls to gateA20_set().

The old warnings about avoiding automatically enabling A20 are
essentially redundant; they date back to the time when we would always
start hammering the keyboard controller without first checking to see
if gate A20 was already enabled (which it almost always is).

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

src/arch/i386/interface/pxeparent/pxeparent.c

@@ -147,16 +147,6 @@ int pxeparent_call ( SEGOFF16_t entry, unsigned int function,
 			         "D" ( __from_data16 ( &pxeparent_params ) )
 			       : "ecx", "edx", "esi", "ebp" );
 
-	/* PXE API calls may rudely change the status of A20 and not
-	 * bother to restore it afterwards.  Intel is known to be
-	 * guilty of this.
-	 *
-	 * Note that we will return to this point even if A20 gets
-	 * screwed up by the parent PXE stack, because Etherboot always
-	 * resides in an even megabyte of RAM.
-	 */
-	gateA20_set();
-
 	/* Determine return status code based on PXENV_EXIT and
 	 * PXENV_STATUS
 	 */