build: make device-tree arg optional in mkits.sh

mkits.sh help currently shows dtb file as optional, but generates an
invalid its file when this option is unspecified.

We're fixing this problem by removing the use of the sed command and
just including variables instead.

Signed-off-by: Mathieu Olivari <mathieu@qca.qualcomm.com>

SVN-Revision: 45659