init: rename init_first_stage

In preparation for splitting first stage init from the rest of the
init executable, rename these files which are specifically involved in
first stage mount operations to a more appropriate name.

Test: builds
Change-Id: I8a2d4e8c7e1deea1bab45cc8e738727bc2ceb3e5
diff --git a/init/init.cpp b/init/init.cpp
index 686cd6e..5352450 100644
--- a/init/init.cpp
+++ b/init/init.cpp
@@ -48,8 +48,8 @@
 
 #include "action_parser.h"
 #include "epoll.h"
+#include "first_stage_mount.h"
 #include "import_parser.h"
-#include "init_first_stage.h"
 #include "keychords.h"
 #include "log.h"
 #include "property_service.h"