Update build for latest Zephyr

The toolchain selection env var has been renamed
ZEPHYR_TOOLCHAIN_VARIANT, update the README.

Including boilerplate.cmake directly is deprecated, use
find_package(Zephyr).

Signed-off-by: Andrew Davis <afd@ti.com>

README.md

@@ -94,7 +94,7 @@ running a Zephyr environment.
 As Zephyr uses CMake, we build libmetal library and test application as targets
 of Zephyr CMake project. Here is how to build libmetal for Zephyr:
 ```
-    $ export ZEPHYR_GCC_VARIANT=zephyr
+    $ export ZEPHYR_TOOLCHAIN_VARIANT=zephyr
     $ export ZEPHYR_SDK_INSTALL_DIR=<where Zephyr SDK is installed>
     $ source <git_clone_zephyr_project_source_root>/zephyr-env.sh
 

cmake/syscheck.cmake

@@ -5,7 +5,7 @@ if (WITH_ZEPHYR)
   string (TOLOWER "Zephyr"                PROJECT_SYSTEM)
   string (TOUPPER "Zephyr"                PROJECT_SYSTEM_UPPER)
   if (NOT WITH_ZEPHYR_LIB)
-    include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
+    find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
   endif()
   # map zephyr arch to libmetal machine
   set (MACHINE "${CONFIG_ARCH}" CACHE STRING "")