BuildSystem: Add Memtest86+ as a secondary payload

This allows memtest86+ to be added to CBFS as a 'secondary'
payload on x86 systems, to be loaded by the main payload
if desired.

Selecting this option, which defaults to no, builds the memtest86+
payload and adds it to CBFS as `img/memtest` which can then be
loaded by for example SeaBIOS or GRUB.

Change-Id: Iecf876aaf588ba1df7abdf6668cb26f089bf5f42
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13858
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Ben Gardner <gardner.ben@gmail.com>
4 files changed