Move target identification code to a separate file

For some cross-platform examples, we need to tweak things based on the target platform. For this reason, target identification needs to happen early, before inclusion of the top-level Makefile.include. Up to now, we have simply replicated the target-identification code to example Makefiles, leading to code-replication and on occasion bugs (incorrect platform identification).

This commits moves the correct working code to a separate file, so that example Makefiles can include this "as a library". The top-level Makefile merely includes this file now.

Makefile.identify-target

@@ -0,0 +1,9 @@
+ifeq ($(TARGET),)
+  -include Makefile.target
+  ifeq ($(TARGET),)
+    ${info TARGET not defined, using target 'native'}
+    TARGET=native
+  else
+    ${info using saved target '$(TARGET)'}
+  endif
+endif

Makefile.include

@@ -8,15 +8,7 @@ endif
 # Setting this option is also important for tests on Cooja motes to check for warnings.
 WERROR ?= 1
 
-ifeq ($(TARGET),)
-  -include Makefile.target
-  ifeq ($(TARGET),)
-    ${info TARGET not defined, using target 'native'}
-    TARGET=native
-  else
-    ${info using saved target '$(TARGET)'}
-  endif
-endif
+include $(CONTIKI)/Makefile.identify-target 
 
 ifeq ($(DEFINES),)
   -include Makefile.$(TARGET).defines