3.14 is used and tested by linux mini-product
to fix
```
CMake Error at CMakeLists.txt:4 (cmake_minimum_required):
Compatibility with CMake < 3.5 has been removed from CMake.
Update the VERSION argument <min> value. Or, use the <min>...<max> syntax
to tell CMake that the project requires at least <min> but has been updated
to work with policies introduced by <max> or earlier.
Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.
```
Upgrade `cmake_minimum_required` from `(VERSION 2.8)` to `(VERSION 2.9)` to
yield the warning:
"Compatibility with CMake < 2.8.12 will be removed from a future version of CMake"
Add "-Wno-unused" for CMAKE_CXX_FLAGS to yield the compilation warnings
when build LLVM JIT.
Fix the link error when code coverage is enabled.
Apply clang-format for C source files in folder core/app-mgr,
core/app-framework, and test-tools.
And rename folder component_test to component-test, update
zephyr build document.
Signed-off-by: Wenyong Huang <wenyong.huang@intel.com>
