tinydtls: updated to latest version with cross-compilation fixes

This update enables cross-compilation by specifying the option
--host to configure. For example, building libcoap with tinydtls
for raspberry pi now can be done as follows (given that you have
the required compiler; on Debian, apt-get install gcc-arm-linux-gnueabihf
should do):

  git submodule update --init --recursive
  ./autogen.sh
  ./configure --host=arm-linux-gnueabihf --disable-shared --disable-documentation
  make