build/roms: Don't build Memtest86+ when not specified by cmdline

When overriding which payloads will be built with the -p command line
argument, the roms_helper script builds the Memtest86+ payload before
checking if it should be disabled. Move the build command after the
command line override.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>