Fix CMake problems in system test

The paths were wrong in the scripts used to run tests. The same problem
was in the CMake files that used CMAKE_SOURCE_DIR instead of
CMAKE_CURRENT_SOURCE_DIR.

Added missing check for BUILD_SYSTEM_TESTS in avrorouter for the
workaround to building without all dependencies present.

cmake/FindLibUUID.cmake

@@ -5,13 +5,13 @@
 
 find_library(LIBUUID_LIBRARIES NAMES uuid)
 if (NOT LIBUUID_LIBRARIES)
-  message(FATAL_ERROR "Could not find libuuid library")
+  message(STATUS "Could not find libuuid library")
 else()
   message(STATUS "Found libuuid ${LIBUUID_LIBRARIES}")
 endif()
 find_path(LIBUUID_HEADERS uuid.h PATH_SUFFIXES uuid/)
 if (NOT LIBUUID_HEADERS)
-  message(FATAL_ERROR "Could not find libuuid headers")
+  message(STATUS "Could not find libuuid headers")
 else()
   message(STATUS "Found libuuid headers ${LIBUUID_HEADERS}")
 endif()