[bootserver][images] Give kernel bootserver switch as --boot.

Currently, the kernel corresponds to a positional bootserver argument,
represented by "" in images.json. This fixes that to pass the kernel
under --boot.

Test: images.json as expected (no ""'s); pave.sh and netboot.sh as
expected.

Change-Id: I5a4f786278b75114e234624a65036677d762fab7