Integrate mbedTLS library to CHIP

[pan-apple]

Problem

CHIP project is lacking an implementation of mbedTLS.

Proposed Solution

There are the following solutions. We need to choose one of them

1. nRF52, and EFR32 SDKs provide an implementation of mbedTLS. Use that for specific platforms. For native (Ubuntu/Darwin), use installed (apt-get/brew) version of mbedTLS.
2. Clone https://github.com/ARMmbed/mbedtls to third_party/mbedTLS and build it for each platform.

[pan-apple]

@rwalker-apple , @gerickson , @woody-apple any thoughts on this?

[rwalker-apple]

since we have autoconf, I recommend option 1

[gerickson]

As a first pass, I'd take the same approach here that we take with OpenSSL, I think. Depending on whether it is required or not, add an NL_WITH_{OPTIONAL,REQUIRED}_EXTERNAL_PACKAGE to configure.ac. This means the system integrator needs to provide it.

If that's insufficient and we find we really need to, we can either make mbedTLS a repo.conf entry, snapping to a particular branch/commit/tag. This means the CHIP package can provide it by "reference" by default, but the system integrator can override it as above.

If that's insufficient, we can actually inline a copy. This means the CHIP package can provide it by "value" by default, but the system integrator can override it as above.

[pan-apple]

Thanks @rwalker-apple , @gerickson. I'll go with the approach suggested above (option 1, with similar setup as OpenSSL as REQUIRED external package).

[pan-apple]

Seems mbedTLS does not have .pc file required by pkg-config. They have an issue opened ~5 years ago.

https://github.com/ARMmbed/mbedtls/issues/228

So pkg-config does not detect mbedtls installation