healthd: create healthd_board_init()

Subsume healthd_board_poll_intervals, make clear the call is for init-time
actions.

Change-Id: I9267e4ce7c62b78d2997a43822f20bfa13b54cd8
diff --git a/healthd/healthd_board_default.cpp b/healthd/healthd_board_default.cpp
index 82f37fe..b2bb516 100644
--- a/healthd/healthd_board_default.cpp
+++ b/healthd/healthd_board_default.cpp
@@ -16,7 +16,7 @@
 
 #include <healthd.h>
 
-void healthd_board_poll_intervals(int *fast_interval, int *slow_interval)
+void healthd_board_init(struct healthd_config *config)
 {
     // use defaults
 }