[prefix] Add A20-enabling code in libflat

iPXE currently insists on residing in an even megabyte.  This imposes
undesirably severe constraints upon our PMM allocation strategy, and
limits our options for mechanisms to access ROMs greater than 64kB in
size.

Add A20 handling code to libflat so that prefixes are able to access
memory even in odd megabytes.

The algorithms and tuning parameters in the new A20 handling code are
based upon a mixture of the existing iPXE A20 code and the A20 code
from the 2.6.32 Linux kernel.

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

src/arch/i386/prefix/libprefix.S

@@ -512,13 +512,24 @@ install_prealloc:
 
 	/* Open up access to payload */
 #ifndef KEEP_IT_REAL
-	/* Flatten real mode */
+	/* Access high memory */
 	pushw	%cs
 	pushw	$1f
 	pushw	%ax
-	pushw	$flatten_real_mode
+	pushw	$access_highmem
 	lret
-1:
+1:	/* Die if we could not access high memory */
+	jnc	3f
+	movw	$a20_death_message, %si
+	xorw	%di, %di
+	call	print_message
+2:	jmp	2b
+	.section ".prefix.data", "aw", @progbits
+a20_death_message:
+	.asciz	"Gate A20 stuck - cannot continue\n"
+	.size	a20_death_message, . - a20_death_message
+	.previous
+3:
 #endif
 
 	/* Calculate physical address of payload (i.e. first source) */
@@ -570,13 +581,13 @@ install_prealloc:
 	popl	%edx /* discard */
 
 	/* Copy code to new location */
+	pushl	%edi
 	xorw	%ax, %ax
 	movw	%ax, %es
-	movl	%ebp, %edi
 	es rep addr32 movsb
+	popl	%edi
 
 	/* Initialise librm at new location */
-	movl	%ebp, %edi
 	lcall	*init_librm_vector
 #endif