Merge pull request #6 from wens/fdt-no-compatible

Add note for fdt image to not derive compatible from fdt or model

Author: sjg20

source/chapter2-source-file-format.rst

@@ -355,6 +355,10 @@ compatible
                                 UltraScale+ (ZynqMP) device.
     ==========================  =========================================
 
+    *Note* For fdt images, the node should not have a compatible for the model.
+    The compatible here is not derived from the fdt, nor is it used to identify
+    the fdt. Such usage belongs in the configuration node.
+
 phase
     U-Boot phase for which the image is intended.