Re-org the IEC repo to adapt to actual deployment

1. Setup a src folder to include all src files
for IEC;
2. Move installation scripts to src/foundation
sub-folder;
3. Add use_cases sub-folder for all possible
IEC use cases;
4. Add SEBA-on-arm as one of the IEC use cases;
other use cases could be added in the future;
5. Add doc/ folder to contain docs for IEC;
other folders could be added in the iec/ root
folder in the future;
6. Add a README file for SEBA-on-arm use case,
please see it for the structure of the
seba_on_arm/ folder;

Change-Id: Ie0fc876e91188c0611dcab953f7ba27f1f353530
Signed-off-by: trevortao <trevor.tao@arm.com>