CI: Exclude MbedTLS subtree for CONFIG checks

CI: Exclude MbedTLS subtree for CONFIG checks

Since MbedTLS is an external repo with its own coding style,
exclude it from Azure and gitlab CI CONFIG checks.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>