samples: intel_s1000_crb: set BOARD for dmic sample

Set BOARD variable in CMakeLists.txt for DMIC Sample App
Since this sample app is for intel_s1000_crb, setting BOARD
in CMakeLists.txt will eliminate the need to specify in
cmake command line every time during build.

Signed-off-by: Sathish Kuttan <sathish.k.kuttan@intel.com>