buildroot: Add ssh connectivity

Can now ssh in as root with no password. On the FVP this requires
either user networking or a tap interface. Uses haveged for entropy
and, as such, startup time is not noticably changed.

On FVP when using user networking connect using:
ssh -o StrictHostKeyChecking=no -p 8022 root@localhost

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Change-Id: Ib01fc525a3e2364aeeee3e5f2d8af07c8c199262