zephyr/CMakeLists: Update for latest Zephyr CMake usage refactorings.

Added cmake_minimum_required and updated target_link_libraries directives.

ports/zephyr/CMakeLists.txt

+cmake_minimum_required(VERSION 3.8)
+
 include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
 project(NONE)
 
@@ -5,7 +7,7 @@ target_sources(app PRIVATE src/zephyr_start.c src/zephyr_getchar.c)
 
 add_library(libmicropython STATIC IMPORTED)
 set_target_properties(libmicropython PROPERTIES IMPORTED_LOCATION ${CMAKE_CURRENT_SOURCE_DIR}/libmicropython.a)
-target_link_libraries(app libmicropython)
+target_link_libraries(app PUBLIC libmicropython)
 
 zephyr_get_include_directories_for_lang_as_string(C includes)
 zephyr_get_system_include_directories_for_lang_as_string(C system_includes)