board: cavs15: Add a option to control signing ways

Zephyr testcases(not SOF case) not use kernel DSP driver to load image
on ADSP board, thus do not need signing with xman. So add a input
'--no-manifest' to specify signing without xman in image. If use DSP
driver load image, we should not specify this.

Signed-off-by: Jian Kang <jianx.kang@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>