zephyr: Use cmake find_package to locate zephyr.

Updates the zephyr port to use the ZEPHYR_BASE environment variable only to
locate the zephyr cmake package, allowing cmake to cache the variable.

ports/zephyr/CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.13.1)
 
-include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
 project(NONE)
 
 target_sources(app PRIVATE src/zephyr_start.c src/zephyr_getchar.c)