nano_init: force nanokernel stack alignment

The interrupt stack pointer is now aligned, and we error out if
the sizes of the interrupt and main task stacks aren't a multiple
of the stack alignment.

Change-Id: I2a70c82fc94e25cc6c7a9d5ec165bf2370b8a166
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>