schema: factor out properties into legacy.json

The current Exposes element schema describes an object composed of
arbitrary properties from a list.  The list of allowed properties is
duplicated; there are already miscompares between the two - so factor
the list out, move it to a common place and reference the common list.

Further, schemas composed of arbitrary properties are too relaxed - more
descriptive schemas that restrict elements to better defined types is
conceiveable, so call the existing property list an "Aggregate" to
highlight its fuzzy nature, and move it to an isolated schema document
so we can add documentation that discourages adding new properties to
the Aggregate schema.

Change-Id: I97df88e0a40fd6fdf1d53b69e2855d31076fa09d
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meson.build b/meson.build
index 584c0f8..2425696 100644
--- a/meson.build
+++ b/meson.build
@@ -112,6 +112,7 @@
 
 schemas = [
     'global.json',
+    'legacy.json',
     'Pid.json',
     'Pid.Zone.json',
     'Stepwise.json',