core: Introduce first_cpu

Extracted from patch of Antonios Motakis: Avoid the barely readable
"for_each_cpu(...) break;" pattern.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

hypervisor/include/jailhouse/control.h

@@ -33,6 +33,14 @@ extern struct jailhouse_system *system_config;
 unsigned int next_cpu(unsigned int cpu, struct cpu_set *cpu_set,
 		      int exception);
 
+/**
+ * Get the first CPU in a given set.
+ * @param set		CPU set.
+ *
+ * @return First CPU in set, or max_cpu_id + 1 if the set is empty.
+ */
+#define first_cpu(set)		next_cpu(-1, (set), -1)
+
 /**
  * Loop-generating macro for iterating over all CPUs in a set.
  * @param cpu		Iteration variable holding the current CPU ID