Add Nlohmann JSON library

This will make use of the Nlohmann JSON library accessible in the
sysroot for other packages to compile with.

Use: #include "nlohmann/json.hpp"

Change-Id: Ia1dc032bc847fa156c0f7c12afb117e2ae3c2192
Signed-off-by: David Cobbley <david.j.cobbley@linux.intel.com>