fix Makefile chmod (644)
replace MD5SUM with HASH
add PKG_MIRROR_HASH when PKG_SOURCE_PROTO:=git

(PKG_SOURCE_PROTO:=svn tarballs are not reproducible for now)

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

From the Tor project page:

obfsproxy is a tool that attempts to circumvent censorship, by
transforming the Tor traffic between the client and the bridge. This
way, censors, who usually monitor traffic between the client and the
bridge, will see innocent-looking transformed traffic instead of the
actual Tor traffic.

This depends on:

- pyptlib (#2053)
- twisted (#2052)

Also, txsocksx (#2058) is necessary to use an outgoing SOCKS proxy,
and having either gmpy2 (#2067) or gmpy (#2051) installed will help
speed up calculations.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>