Better building of tinc from source

Changes:
* Use openssl and tinc as git submodules (safer, easier, stays between builds and checkouts)
* Don't just "git clone" tinc, which meant using master
* Don't include res/raw/tinc in the repo
* Clean up the Makefile of other unnecessary stuff like the downloading of the NDK

That last bit is secondary, but I still think that having it clutters the Makefile.

Removing the tinc binary from the repo might be a step backwards for out-of-the-box building for newbies, but if you include instructions explaining to do a "make -C src_tinc/ all install", it should still be easy enough without having to depend on prebuilts. 

From now on, if you want to update the version of tinc or openssl that you are using, it's as easy as updating the submodules to a newer git tag. You don't have to worry about package urls nor hash sums.