[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/image/nbi.c

@@ -1,7 +1,6 @@
 #include <errno.h>
 #include <assert.h>
 #include <realmode.h>
-#include <gateA20.h>
 #include <memsizes.h>
 #include <basemem_packet.h>
 #include <ipxe/uaccess.h>
@@ -306,8 +305,6 @@ static int nbi_boot16 ( struct image *image, struct imgheader *imgheader ) {
 	       imgheader->execaddr.segoff.segment,
 	       imgheader->execaddr.segoff.offset );
 
-	gateA20_unset();
-
 	__asm__ __volatile__ (
 		REAL_CODE ( "pushw %%ds\n\t"	/* far pointer to bootp data */
 			    "pushw %%bx\n\t"
@@ -327,8 +324,6 @@ static int nbi_boot16 ( struct image *image, struct imgheader *imgheader ) {
 		  "b" ( __from_data16 ( basemem_packet ) )
 		: "ecx", "edx", "ebp" );
 
-	gateA20_set();
-
 	return rc;
 }
 
@@ -345,8 +340,6 @@ static int nbi_boot32 ( struct image *image, struct imgheader *imgheader ) {
 	DBGC ( image, "NBI %p executing 32-bit image at %lx\n",
 	       image, imgheader->execaddr.linear );
 
-	/* no gateA20_unset for PM call */
-
 	/* Jump to OS with flat physical addressing */
 	__asm__ __volatile__ (
 		PHYS_CODE ( "pushl %%ebx\n\t" /* bootp data */