upload_manifest: strip objects included in images

With this patch we strip all .so included in our build images,
regardless if it's a debug or a release build.

Basically any object ended with .so is stripped and sent to the
image without the -strip suffix. The code will check if there isn't
already an stripped object and if timestamp matches it, so it won't
strip unecessary files.

Everything outside build/target is ignored.

Fixes #133

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Eduardo Piva <efpiva@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>