base: add util/dictionary.h utility

The new 'Dictionary' provides an easy way to access objects using
strings as key. The 'String' received the 'operator >' to simplify the
organization of strings in an AVL tree.

The patch removes the former definition of the 'operator >' from the
platform driver because it would be ambigious now.

Fixes #4610

repos/base/include/util/string.h

@@ -808,6 +808,12 @@ class Genode::String
 			return strcmp(string(), other.string()) != 0;
 		}
 
+		template <size_t N>
+		bool operator > (String<N> const &other) const
+		{
+			return strcmp(string(), other.string()) > 0;
+		}
+
 		void print(Output &out) const { Genode::print(out, string()); }
 };