Build examples and tests with -Werror (BSP-176)

[igrr]

To make sure that the examples can be built without warnings, we should export EXTRA_CFLAGS and EXTRA_CXXFLAGS variables in CI before running idf.py build.

Here's a snippet from IDF:

export PEDANTIC_FLAGS="-DIDF_CI_BUILD -Werror -Werror=deprecated-declarations -Werror=unused-variable -Werror=unused-but-set-variable -Werror=unused-function"
export PEDANTIC_CFLAGS="${PEDANTIC_FLAGS} -Wstrict-prototypes"
export PEDANTIC_CXXFLAGS="${PEDANTIC_FLAGS}"
export EXTRA_CFLAGS=${PEDANTIC_CFLAGS}
export EXTRA_CXXFLAGS=${PEDANTIC_CXXFLAGS}

(IDF build system picks up EXTRA_CFLAGS and EXTRA_CXXFLAGS environment variables)

• Test app pedantic flags
• Examples pedantic flags

[VojtechBartoska]

@tore-espressif there have been 2 PRs merged, can this issue be closed as solved? Thanks