[tinydtls] Makefile: Updated PKG_URL
The tinydtls repository has moved from Eclipse Gerrit to GitHub [1].
PKG_URL now points to the new direction and PKG_VERSION was updated
to the HEAD of the develop branch.
[1] https://github.com/eclipse/tinydtls.git
The integration of TinyDTLS for RIOT has been merged into the
development branch of the official repository.
It will be officially available on the master branch in the next
release of tinyDTLS.
Other minor updates:
- Support for the RIOT's memarray
- tinydtls/sha2 is removed from the compilation for giving priority
to RIOT's sha2 functions.
