zephyr: reference local Kconfig

Reference Kconfig in zephyr module file which is evaluated by Zephyr
directly. This will eliminate the need for including the SOF Kconfig
directly.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

zephyr/Kconfig

@@ -0,0 +1,3 @@
+if SOF
+rsource "../Kconfig"
+endif

zephyr/module.yml

@@ -1,2 +1,3 @@
 build:
-        cmake: ./zephyr
+  cmake: ./zephyr
+  kconfig: ./zephyr/Kconfig