fix build from tail_build

gb_Side is set either from toplevel Makefile or from any module Makefile,
but not when starting build from tail_build

Change-Id: I4df6eb221f5b778ce3c0234251f8dbbb56645028

tail_build/Makefile

@@ -8,7 +8,10 @@
 #
 
 ifeq ($(strip $(SOLARENV)),)
-include $(dir $(realpath $(firstword $(MAKEFILE_LIST))))/../config_host.mk
+ifeq ($(gb_Side),)
+gb_Side:=host
+endif
+include $(dir $(realpath $(firstword $(MAKEFILE_LIST))))/../config_$(gb_Side).mk
 endif
 
 include $(SOLARENV)/gbuild/gbuild.mk