cmake: CMake 3.13 doesn't like INTERFACE with target_link_libraries

[yashi]

Describe the bug
CMake 3.13 start complaining about CMP0079: target_link_libraries() allows use with targets in other directories.

To Reproduce
Just make for some board.

Expected behavior
No warning while build

Impact
It builds fine but prints the warning a few times.

Screenshots or console output

CMake Warning (dev) at zephyr/cmake/extensions.cmake:837 (target_link_libraries):
  Policy CMP0079 is not set: target_link_libraries allows use with targets in
  other directories.  Run "cmake --help-policy CMP0079" for policy details.
  Use the cmake_policy command to set the policy and suppress this warning.

  Target

    zephyr_interface

  is not created in this directory.  For compatibility with older versions of
  CMake, link library

    -ldl

  will be looked up in the directory in which the target was created rather
  than in this calling directory.
Call Stack (most recent call first):
  zephyr/cmake/extensions.cmake:1055 (target_link_libraries_ifdef)
  zephyr/cmake/extensions.cmake:115 (target_ld_options)
  zephyr/arch/posix/CMakeLists.txt:30 (zephyr_ld_options)
This warning is for project developers.  Use -Wno-dev to suppress it.

Environment (please complete the following information):

• OS: Linux
• Toolchain: CMake 3.13.1
• Commit SHA: d38aa1f

[SebastianBoe]

I believe we want to short-term use the cmake_policy() command to set this policy to OLD, and then later on, when necessary, port the code-base to the NEW behaviour and set the policyt to NEW.

[tejlmand]

I believe we want to short-term use the cmake_policy() command to set this policy to OLD, and then later on, when necessary, port the code-base to the NEW behavior and set the policy to NEW.

basically there is no porting needed if allowing the new behavior, current code works on CMP0079 NEW. The reason to state CMP0079 OLD is in order to still allow users to use CMake versions 3.8.2 - 3.13