CPU affinity configuration fixes

- if no affinity was set for a new thread before calling
  Cpu_session::start(), the CPU session's affinity gets set for this
  thread
- documentation fix: <affinity_space> -> <affinity-space>

Fixes #873.

base-linux/src/core/include/platform_thread.h

@@ -114,7 +114,19 @@ namespace Genode {
 			}
 
 			const char   *name() { return _name; }
-			void          affinity(Affinity::Location) { }
+
+			/**
+			 * Set the executing CPU for this thread
+			 *
+			 * SMP is currently not directly supported on Genode/Linux
+			 * (but indirectly by the Linux kernel).
+			 */
+			void affinity(Affinity::Location) { }
+
+			/**
+			 * Request the affinity of this thread
+			 */
+			Affinity::Location affinity() { return Affinity::Location(); }
 
 			/**
 			 * Register process ID and thread ID of thread