Add retry mechanism to mount and network downloads

Created by: bjoto

When booting from an USB-stick, the STBOOT and STDATA partition wont be available immediately, and as a result stboot will abort and reset the system. The problem is at the kernel has not reached the point, where the USB-stick can be mounted.

To work-around this, add a retry mechanism around the mount function.

Also, remove the timeout parameter, which was not implemented.

Further; Add a similar retry mechanism to network downloads. Somethings getting the link up on a port takes up to a couple of seconds. Retry, instead of failing immediately.