Clean up the Makefile to modern standards.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

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>

Signed-off-by: Alexander Ryzhov <openwrt@ryzhov-al.ru>

Signed-off-by: Luka Perkov <luka@openwrt.org>