sdcard_image-sunxi.bbclass: fixing race condition with virtual/kernel (r...

...esolves #15)

Fixing a race condition with virtual/kernel:do_deploy,
which needs to deploy the image before the IMAGE_CMD_sunxi-sdimg () code is being run
as part of image:do_rootfs, otherwise the following error would sometimes occur
when compiling with multiple threads (BB_NUMBER_THREADS=2):
"[..]/deploy/images/cubieboard2/uImage-cubieboard2.bin: No such file or directory"

Fixing this by using the kernel image in sysrootfs instead.

At the same time, also adding an explicit rootfs dependency on sunxi-board-fex:do_deploy because of
"${DEPLOY_DIR_IMAGE}/fex.bin", although this never seems to pose problems in practice.