examples/power: add JSON string handling

examples/power: add JSON string handling

Add JSON string handling to vm_power_manager for JSON strings received
through the fifo. The format of the JSON strings are detailed in the
next patch, the vm_power_manager user guide documentation updates.

This patch introduces a new dependency on Jansson, a C library for
encoding, decoding and manipulating JSON data. To compile the sample app
you now need to have installed libjansson4 and libjansson-dev (these may
be named slightly differently depending on your Operating System)

Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>