meson: add nlohmann-json wrapper and deps

A number of locations within the repository do not appropriately
specify the dependency on nlohmann-json.  Fix up the wrap file,
add appropriate dependency detection in the root meson, and add the
library as a dependency when needed.  This gets closer to allowing
full subproject-based builds.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If70888c6de5651ca355f84616e41408ddde9fc08
diff --git a/test/meson.build b/test/meson.build
index 3306a91..3fb652b 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -27,7 +27,7 @@
     include_directories: root_inc,
     build_by_default: false,
     implicit_include_directories: false,
-    dependencies: [entity_map_json_dep, gtest, gmock]
+    dependencies: [entity_map_json_dep, gtest, gmock, nlohmann_json_dep]
   ))
 
 # Build/add oemrouter_unittest to test suite