runqemu: add DEPLOY_DIR_IMAGE replacement in QB_OPT_APPEND

Add the ability to replace DEPLOY_DIR_IMAGE with that path in
QB_OPT_APPEND.  This allows for anything present in DEPLOY_DIR_IMAGE to
be passed into the qemu parameters.  This is especially useful if you
want to run multiple flash images (as -bios only allows for one).

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>