Fix warning due to missing file for bbb-swu-image

bbb-swu-image is an example using the swupdate class.
Generally, this recipe should not belong to meta-swupdate,
but it is here as reference for an implementation.

Building with other machines as beaglebone raises
a warning because sw-description is not found.

Signed-off-by: Stefano Babic <sbabic@denx.de>

recipes-extended/images/bbb-swu-image.bb

@@ -5,7 +5,7 @@ DESCRIPTION = "Example Compound image for beaglebone "
 SECTION = ""
 
 # Note: sw-description is mandatory
-SRC_URI = "file://sw-description \
+SRC_URI_beaglebone = "file://sw-description \
            "
 inherit swupdate