Detect if nrfjprog is a recognised command

[gudnimg]

Currently the code does not check whether or not nrfjprog is actually included in PATH. One way to check if to see if nrfjprog.exe is found using find_program()

There's also no gurantee that if a user defines a path for NRF5_NRFJPROG that it is a valid path.

if(NOT NRF5_NRFJPROG)
    # Check if nrfjprog command is recognised
    find_program(NRFJPROG "nrfjprog")
    if(NRFJPROG)
        set(NRF5_NRFJPROG "nrfjprog" CACHE FILEPATH "" FORCE)
        message(STATUS "Using nrfjprog utility available from PATH: ${NRF5_NRFJPROG}")
    else()
        message(FATAL_ERROR "Could no find nrfjprog utlity from PATH")
    endif()
else()
  message(STATUS "Using nrfjprog utility: ${NRF5_NRFJPROG}")
endif()