wic: set bootimg_dir when using image-name artifacts

wic: set bootimg_dir when using image-name artifacts

Running wic with -e to use artifacts from a named image, bootimg_dir was
always passed as empty string to partition source plugins. The patch
sets bootimg_dir to current value of DEPLOY_DIR_IMAGE, as bootloader
artifacts end up in that location as well.

(From OE-Core rev: d7f69e6f0932a927b6ce289fb47ba575d7aaa1c8)

Signed-off-by: Maciej Borzecki <maciej.borzecki@open-rnd.pl>
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>