arm:trats:fix: Correction of loaddtb environment variable.

Missing space for loaddtb command has been added. When missing, ext4load
fails with wrong number of passed parameters.

Acked-by: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
mkimage: Use board config to get CONFIG_FIT_SIGNATURE value

The value of this config variable is not available to image.h on the host,
since the board config is not actually included. Bring this in so that
mkimage will be built with image-signing support for sandbox at least.

Signed-off-by: Simon Glass <sjg@chromium.org>