build-aux: Use Mbed TLS for libdatachannel

[Sean-Der]

Description

Switch to Mbed TLS for libdatachannel in flatpak

Motivation and Context

Resolves #9846 WHIP now works in flatpak

This also matches how libdatachannel is used on other platforms (mac OS)

How Has This Been Tested?

Locally when I use WHIP it works

Types of changes

• Bug fix (non-breaking change which fixes an issue) -->

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[Sean-Der]

@pkviet @tt2468

[tytan652]

Resolves #9846 WHIP now works in flatpak

This also matches how libdatachannel is used on other platforms (mac OS)

Missing explanation on why the OpenSSL backend on Flatpak causes issues, matching (and it just works) is not good as only motivation.

[Sean-Der]

@tytan652 Here is the PR upstream paullouisageneau/libdatachannel#1075

The version of OpenSSL currently used doesn't return a curve for P-256, only prime256v1

I will land the change upstream and then update obs-deps

[Sean-Der]

Closing. Instead opened a PR upstream against libdatachannel to use different curve alias