libsrtp: use OpenSSL

[micmac1]

Use OpenSSL instead of the internal crypto backend. Everything in OpenWrt that links to libsrtp2 depends on OpenSSL anyway.

Upsides:

• the libsrtp2 package size shrinks a bit (for example from 35 to 24 KiB on ath79)
• allows to use more cipher suites
• may allow for hardware acceleration

Closes #763

Signed-off-by: Sebastian Kemper sebastian_ml@gmx.net

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Maintainer: @jslachta Compile tested: 22.03-rc1 ath79 Run tested: 22.03-rc1 ath79, made some calls using SRTP (sdes)

Description: Hi all,

PR to fix the recently opened issue.

Kind regards, Seb