Use our own mbedtls library.

slapos/software/osie-coupler/software-opc-ua.cfg

 [buildout] 
 parts = 
+  mbedtls
   open62541
   compile-coupler
   slapos-cookbook
@@ -11,6 +12,8 @@ extends =
   https://lab.nexedi.com/nexedi/slapos/raw/master/component/cmake/buildout.cfg
   https://lab.nexedi.com/nexedi/slapos/raw/master/component/git/buildout.cfg
   https://lab.nexedi.com/nexedi/slapos/raw/master/component/python3/buildout.cfg
+  https://lab.nexedi.com/nexedi/slapos/raw/master/component/mbedtls/buildout.cfg
+
 
 # we need to have bootstrap Python for buildout in Python3 due to
 # https://bugs.kde.org/show_bug.cgi?id=344802
@@ -56,6 +59,7 @@ environment =
   OPEN62541_HOME = ${open62541:location}
   OPEN62541_SOURCE_HOME = ${open62541-source:location}
   BINARY_OUT_DIR = ${buildout:directory}/bin
+  EXTRA_FLAGS = -L ${mbedtls:location}/lib  -lmbedtls -lmbedx509 -lmbedcrypto
 # actually configure-command is useless here but needed by convention
 configure-command = 
   ${cmake:location}/bin/cmake