Revert "mb/aopen/dxplplusu: Remove board"

This reverts commit eb76a455cd39ec59b7f2ba28baeec9538befd59e
and applies minor fixes to make it build again.

PARALLEL_MP was working prior to board removal and no
relevant SMI handlers were implemented. So NO_SMM choice
is now selected.

Change-Id: Ia1cd02278240d1b5d006fb2a7730d3d86390f85b
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69339
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/src/mainboard/aopen/dxplplusu/Makefile.inc b/src/mainboard/aopen/dxplplusu/Makefile.inc
new file mode 100644
index 0000000..d604d02
--- /dev/null
+++ b/src/mainboard/aopen/dxplplusu/Makefile.inc
@@ -0,0 +1,3 @@
+## SPDX-License-Identifier: GPL-2.0-only
+
+bootblock-y += bootblock.c