asus/p2b*: Move serial init into mainboard bootblock

With this bootblock messages are transmitted over serial too.

TEST=Serial messages transmitted normally on asus/p2b-ls.

Change-Id: I6f3ee68e7c76a8c6db6d75956e6a7fb75ef83850
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38670
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/src/mainboard/asus/p2b/Makefile.inc b/src/mainboard/asus/p2b/Makefile.inc
index ca08106..cc55c25 100644
--- a/src/mainboard/asus/p2b/Makefile.inc
+++ b/src/mainboard/asus/p2b/Makefile.inc
@@ -1,2 +1,4 @@
+bootblock-y += bootblock.c
+
 ramstage-$(CONFIG_GENERATE_PIRQ_TABLE) += variants/$(VARIANT_DIR)/irq_tables.c
 ramstage-$(CONFIG_GENERATE_MP_TABLE) += variants/$(VARIANT_DIR)/mptable.c