contiki: work without running configure
When building with contiki, 'CONTIKI' is defined and is used to detect contiki.
Configure options can be configured with:
DTLS_CONF_ECC, DTLS_CONF_PSK, DTLS_CONF_NDEBUG
All files now include 'tinydtls.h' instead of 'dtls_config.h'.
Change-Id: I9a558a7d6bd777656d7b29028852bfc08911b8b7
Signed-off-by: Gaëtan Harter <gaetan.harter@inria.fr>