Add to support mbed TLS 2.1.x encryption backend

Hi

As you know, Zabbix 3.0.x and later support encrypted communication.
Zabbix currently support following encryption backends:

• OpenSSL
• GnuTLS
• mbed TLS (only 1.3.x)

But mbed TLS 1.3.x will drop to support by ARM soon
https://tls.mbed.org/tech-updates/releases/mbedtls-2.6.0-2.1.9-and-1.3.21-released

So, I made the patch to support mbed TLS 2.1.x.

This patch adds new "configure" script option:
--with-mbedtls21[=DIR]

Internally, adding a new preprocessor define (HAVE_MBEDTLS21) and keeping mbed TLS 1.3.x support (HAVE_POLARSSL)

If they keep API compatibility, It probably works with mbed TLS 2.x branch (eg: 2.6.0).

This patch is for "branches/3.0" revision 74336.