Add policy_max to minimum required cmake version

Specify the policy max value on the cmake_minimum_required, that allows CMake
to use newer CMake policy up to CMake 3.16.

CMakeLists.txt

@@ -1,6 +1,6 @@
 # Many projects still are stuck using CMake 2.8 is several places so it's good to provide backward support too. This is
 # specially true in old embedded systems (OpenWRT and friends) where CMake isn't necessarily upgraded.
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 2.8...3.16)
 
 if(POLICY CMP0048)
 	cmake_policy(SET CMP0048 NEW)

apps/CMakeLists.txt

@@ -1,5 +1,5 @@
 
-cmake_minimum_required(VERSION 2.8)  # see ../CMakeLists.txt for why 2.8
+cmake_minimum_required(VERSION 2.8...3.16)  # see ../CMakeLists.txt for why 2.8
 
 if(POLICY CMP0075)
     cmake_policy(SET CMP0075 NEW)