[romprefix] Round up PMM allocation sizes to nearest 4kB

Some AMI BIOSes apparently break in exciting ways when asked for PMM
allocations for sizes that are not multiples of 4kB.

Fix by rounding up the image source area to the nearest 4kB.  (The
temporary decompression area is already rounded up to the nearest
128kB, to facilitate sharing between multiple iPXE ROMs.)

Reported-by: Itay Gazit <itayg@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/romprefix.S

 	call	print_message
 	/* We have PMM and so a 1kB stack: preserve whole registers */
 	pushal
-	/* Allocate image source PMM block */
+	/* Allocate image source PMM block.  Round up the size to the
+	 * nearest 4kB (8 512-byte sectors) to work around AMI BIOS bugs.
+	 */
 	movzbl	romheader_size, %ecx
 	addw	extra_size, %cx
+	addw	$0x0007, %cx	/* Round up to multiple of 8 512-byte sectors */
+	andw	$0xfff8, %cx
 	shll	$5, %ecx
 	movl	$PMM_HANDLE_BASE_IMAGE_SOURCE, %ebx
 	movw	$get_pmm_image_source, %bp