Link to system mbedtls

[Oxymoron79]

This PR adds a BUILD_MBEDTLS CMake option:

• If set to ON (default) the mbedtls library is patched and built by the iotivity-lite build system.
• If set to OFF, the iotivity-lite build system searches for the mbedtls library using CMake's find_package function and verifies that the library has been built with the OCF patches applied.

To test this feature, I used patched, built and installed the mbedtls library from the deps/mbedtls submodule before building iotivity-lite:
To patch mbedtls I used the deps/mbedtls-patch.cmake script:

$ cmake -DOC_DYNAMIC_ALLOCATION_ENABLED=ON -DOC_PKI_ENABLED=ON -P deps/mbedtls-patch.cmake

Then I configured, built and installed mbedtls as a shared library:

$ cd deps/mbedtls
$ cmake -DCMAKE_BUILD_TYPE=Release -DENABLE_PROGRAMS=OFF -DENABLE_TESTING=OFF -DUSE_SHARED_MBEDTLS_LIBRARY=ON .
$ make install

After that, with BUILD_MBEDTLS=OFF iotivity-lite was able to

• find the mbedtls shared library with the find_package function
• verify that mbedtls was patched by checking for the mbedtls_x509write_crt_set_subject_alt_names function
• compile iotivity-lite and link to the mbedtls shared library

But to make the patched mbedtls library compile successfully, I had to modify the OCF patches and remove the generic entropy pool setting, since that requires parts of the iotivity-lite sources.

I am not sure what impact this modification of the OCF patches for mbedtls has and whether that would be acceptable.
I also haven't tested whether BUILD_MBEDTLS=OFF works on all platforms and with different build configurations.

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[Danielius1922]

Nice job, Martin!

We've talked with @jkralik and if we understand the problem with the MBEDTLS_ENTROPY_C correct, the issue is that mbedtls_platform_entropy_poll is added by the patch. Which includes port/oc_random.h and uses oc_random_value from iotivity-lite. So we decided that instead of directly using oc_random_value I will update the patch and add a function pointer and a setter for this pointer to mbedtls and this pointer will be used instead of oc_random_value. Iotivity lite will set this function to oc_random_value() when it initializes mbedtls.
Similar issue might be with MBEDTLS_PLATFORM_STD_EXIT which is defined as oc_exit, but we will solve it the same way. And this should allow us to keep MBEDTLS_ENTROPY_C enabled. I'll add commits to this PR if that's OK.

Also to test the various compilation options I'm thinking about modifying the GH workflows. In most workflows we have workflow_dispatch enabled, so one can run a workflow manually. And dispatching can be parameterized. I'll add a bool, which controls whether preinstalled or submodule mbedtls should be used. The workflows will need to be modified to add an mbedtls installation step, but that shouldn't be complicated.

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