[prefix] Use area at top of INT 15,88 memory map for temporary decompression

[prefix] Use area at top of INT 15,88 memory map for temporary decompression

Use INT 15,88 to find a suitable temporary decompression area, rather
than a fixed address.  This hopefully gives us a better chance of not
treading on any PMM-allocated areas, in BIOSes where PMM support
exists but tends not to give us the large blocks that we ask for.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>