image-micro: Don't use direct assign for EXTRA_IMAGEDEPENDS
image-micro: Don't use direct assign for EXTRA_IMAGEDEPENDS

This overwrites any modifications made by machine configs, which is a
valid thing to do.

Signed-off-by: Tom Zanussi <1c8a3d9fe0daebf50077f18cba0f71e4a3d97b0e@linux.intel.com>

tzanussi/meta-micro-galileo,tzanussi/meta-micro-galileo