sdcard_image-rpi.bbclass: Fix when RPI_SDIMG_EXTRA_DEPENDS not defined

If the variable is not defined, bitbake will fail:

[...]
Task 'depends' should be specified in the form 'packagename:task'
[...]

This is because not expanding the variable leaves an invalid entry.

Signed-off-by: Andrei Gherzan <andrei@gherzan.ro>

classes/sdcard_image-rpi.bbclass

@@ -49,6 +49,8 @@ SDIMG_ROOTFS = "${IMGDEPLOYDIR}/${IMAGE_LINK_NAME}.${SDIMG_ROOTFS_TYPE}"
 # For the names of kernel artifacts
 inherit kernel-artifact-names
 
+RPI_SDIMG_EXTRA_DEPENDS ?= ""
+
 do_image_rpi_sdimg[depends] = " \
     parted-native:do_populate_sysroot \
     mtools-native:do_populate_sysroot \