mbedtls: Switch default MBEDTLS_CFG_FILE to config-mini-tls1_2.h

The previous default, "config-threadnet.h", is more or less arbitrary
choice made in a commit 312def2c 1.5 years ago. In particular,
it's not related to Thread support in Zephyr per se (there was no
such support at that time).

It doesn't make sense to have a default intended for a particular,
not widely used (yet) protocol. Instead, the default should work
out of the box with a contemporary widely deployed arrays of TLS
servers, which are HTTPS server. config-mini-tls1_2.h works with
https://google.com

, and by extension, with many other servers on
the Internet.

So, have that as the default, and let applications with special
needs to override that to what they need.

Addresses: #6132

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>