im_n29xx_t40n: determine ONIE install device

At runtime determine the Linux device mapping for the internal mSATA
drive on the T40N.

The approach here is to look for the block device that has a very
specific, bus centric path in the system.  The internal mSATA device
can only show up with this address.

im_n29xx_disk="/sys/devices/pci0000:00/0000:00:11.0/host0/target0:0:0/0:0:0:0"