kernel: add yield syscall

kernel-src/sys/syscalls/yield.c

@@ -0,0 +1,10 @@
+#include <kernel/syscalls.h>
+#include <kernel/scheduler.h>
+
+syscallret_t syscall_yield(context_t *) {
+	syscallret_t ret = {0};
+
+	sched_yield();
+
+	return ret;
+}