[prefix] Default to 1MB mark as fallback high memory load point

Now that we can use odd megabytes, there is no particular need to use
an even megabyte as the fallback temporary load point.

Note that the old warnings about avoiding 2MB pre-date our ability to
cooperate with other PXE ROMs by using PMM.

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

src/arch/i386/prefix/libprefix.S

  * Temporary buffer into which to copy (or decompress) our runtime
  * image, prior to calling get_memmap() and relocate().  We don't
  * actually leave anything here once install() has returned.
- *
- * We use the start of an even megabyte so that we don't have to worry
- * about the current state of the A20 line.
- *
- * We use 4MB rather than 2MB because some PXE stack / PMM BIOS
- * combinations are known to place data required by other UNDI ROMs
- * loader around the 2MB mark.
  */
 	.globl	HIGHMEM_LOADPOINT
-	.equ	HIGHMEM_LOADPOINT, ( 4 << 20 )
+	.equ	HIGHMEM_LOADPOINT, ( 1 << 20 )
 
 /* Image compression enabled */
 #define COMPRESS 1