Makefile.mk: Fix inclusion of site-local/Makefile.inc

In commit 854dd9a5d1153fbb7ace2a7619bb98d024e284ce
(Makefile.mk: Put site-local path first) the inclusion of
site-local/Makefile.inc was moved to the first place. Unfortunately,
the very next line where subdirs-y is modified resets the variable
instead of extending it which overwrites the inclusion of
site-local/Makefile.inc. This breaks setups where
site-local/Makefile.inc is required.

This patch fixes it.

Change-Id: I36ad1aca5742869c84e2fb556f898f896c6f037a
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83190
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Michał Kopeć <michal.kopec@3mdeb.com>
diff --git a/Makefile.mk b/Makefile.mk
index 301a3bc..16b6539 100644
--- a/Makefile.mk
+++ b/Makefile.mk
@@ -99,7 +99,7 @@
 # site-local Makefile.mk must go first to override default locations (for binaries etc.)
 subdirs-y := site-local
 
-subdirs-y := src/lib src/commonlib/ src/console src/device src/acpi src/superio/common
+subdirs-y += src/lib src/commonlib/ src/console src/device src/acpi src/superio/common
 subdirs-$(CONFIG_EC_ACPI) += src/ec/intel
 subdirs-y += src/ec/acpi $(wildcard src/ec/*/*) $(wildcard src/southbridge/*/*)
 subdirs-y += $(wildcard src/soc/*) $(wildcard src/soc/*/common) $(filter-out $(wildcard src/soc/*/common),$(wildcard src/soc/*/*))