u-boot.inc: fix incorrect use of DEPLOY_DIR_IMAGE instead of DEPLOYDIR

the custom do_deploy task incorrectly deploys files into
DEPLOY_DIR_IMAGE instead of DEPLOYDIR. While it seems to work since
the files are actually deployed, it bypasses the sstate, and the
content of the deploy sstate tarball ends up being empty. Instead the
do_deploy should deploy files into DEPLOYDIR, so that whatever is
deployed ends up in the sstate cache. The problem was initially found
by doing the following sequence of operations:

- build an image
- delete TMPDIR
- rebuild the same image (using all content from sstate)

after the rebuild, whatever was done in the u-boot deploy task was
missing in the new image (e.g. MLO, u-boot.img files/symlink).

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>