asus/p2b: Transform into variant-enabled structure

Get ready to squash all the ASUS i440BX boards together.

Change-Id: Ibc9bfa4fc5b582bf658215bda298523e8ee7b36b
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38621
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/src/mainboard/asus/p2b/Makefile.inc b/src/mainboard/asus/p2b/Makefile.inc
new file mode 100644
index 0000000..640396e
--- /dev/null
+++ b/src/mainboard/asus/p2b/Makefile.inc
@@ -0,0 +1 @@
+ramstage-$(CONFIG_GENERATE_PIRQ_TABLE) += variants/$(VARIANT_DIR)/irq_tables.c