Add app2sd-mount-helper tool

Add app2sd-mount-helper tool

When launching an application which is installed at external storage,
app2sd-server mounts application's image at internal storage.
Since process label of app2sd-server is "System", the directory which is
mount point for application's image has same label as app2sd-server.
This can cause smack denial in some cases.

This app2sd-mount-helper tool does mount instead of app2sd-server, and its
process label is "User::Home" which is same as other directories or
files at package root. Using this tool, the smack denial will not
happened.

Change-Id: Ic89bc960372d0a2595cd21aa6d9372473aafc2c8
Signed-off-by: Sangyoon Jang <jeremy.jang@samsung.com>