build: Move BSP_INCLUDES split

The goal is to let the build items define as much as possible.

spec/build/bsps/optincludes.yml

@@ -3,6 +3,7 @@ actions:
 - set-value: |
     bsps/include bsps/${ARCH}/include bsps/${ARCH}/${BSP_FAMILY}/include
 - substitute: null
+- split: null
 - env-assign: null
 build-type: option
 copyrights:

wscript

@@ -615,7 +615,7 @@ class BSPItem(Item):
     def prepare_build(self, bld, bic):
         return BuildItemContext(
             bic.includes
-            + bld.env.BSP_INCLUDES.split()
+            + bld.env.BSP_INCLUDES
             + self.substitute(bld, self.data["includes"]),
             self.substitute(bld, self.data["cppflags"]),
             self.substitute(bld, self.data["cflags"]),