compiling on arm gives zlib errors

It seems the cross compilation toolchain for arm does not contain zlib.a
Hence, for arm, we should compile zlib ourselves.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>
Add subsurface submodule

Rather than manually adding all the subsurface files, it would be better to
include a submodule. This would help in keeping the application up to date with
the upstream.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>