install udev rules on /etc and not ${CMAKE_INSTALL_PREFIX}/etc

This should works fine also with cpack and packaging seen that they
are using DESTDIR.

CPACK:
- from 2.8.3 CPACK_SET_DESTDIR is default to ON
- if you have trouble read CPACK_PACKAGING_INSTALL_PREFIX

Author: ra1nb0w

conf/CMakeLists.txt

@@ -26,7 +26,7 @@ file(WRITE "${CMAKE_BINARY_DIR}/direwolf.conf" "${file_content}")
 
 # install udev rules for CM108
 if(LINUX)
-  install(FILES "${CUSTOM_CONF_DIR}/99-direwolf-cmedia.rules" DESTINATION etc/udev/rules.d/)
+  install(FILES "${CUSTOM_CONF_DIR}/99-direwolf-cmedia.rules" DESTINATION /etc/udev/rules.d/)
 endif()
 
 install(FILES "${CMAKE_BINARY_DIR}/direwolf.conf" DESTINATION share/doc/${CMAKE_PROJECT_NAME}/examples)