Fix building mbed TLS with CMake and allow specifying custom directories

Fix building mbed TLS with CMake and allow specifying custom directories

When installing mbed TLS 2.x and 3.x in parallel, it is useful to point
cmake to the version that should be used.

This fixes also building mbed TLS versions with cmake.

Change-Id: I7fd9e730e87210d2b7d090c8f9c7c6734bd7374e
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20231211170549.85749-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27763.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>