Makefile.include: Fix default RIOTBASE when there is Makefile.local

When an application only includes Makefile.include without specifying
RIOTBASE and uses a `Makefile.local` file, RIOTBASE would use a wrong
default value and get the value of the directory where `Makefile.local` is.

Makefile.include

+# 'Makefile.include' directory, must be evaluated before other 'include'
+_riotbase := $(dir $(lastword $(MAKEFILE_LIST)))
+
 # Globally set default goal to `all`
 .DEFAULT_GOAL := all
 
@@ -5,7 +8,7 @@
 -include Makefile.local
 
 # set undefined variables
-RIOTBASE       ?= $(dir $(lastword $(MAKEFILE_LIST)))
+RIOTBASE       ?= $(_riotbase)
 CCACHE_BASEDIR ?= $(RIOTBASE)
 RIOTCPU        ?= $(RIOTBASE)/cpu
 RIOTBOARD      ?= $(RIOTBASE)/boards