boards: xtensa: Use a CMake variable to set the rimage target name

Removes hardcoded logic in the west signing script that translates
Zephyr board names to rimage target names. Instead, use a cached CMake
variable set at the board level to define its respective rimage target
name. This eliminates the need to modify the west signing script when
new SOF-supported boards are introduced to Zephyr.

Signed-off-by: Maureen Helm <maureen.helm@intel.com>