recovery: Drop the "--stages" / '-g' argument.

This was introduced in commit c87bab101893e8322b49d7c8600e3367b20ab50a.
But the stage info should be passed through BCB only (there's a
dedicated field in struct bootloader_message).

This CL removes it from recovery arguments, and also moves 'stage'
variable to std::string.

Test: 'stage' variable is not used by any device-specific recovery code.
Test: Code search shows no hit of '--stages' use.
Change-Id: Iccbde578a13255f2b55dd4a928e9ecf487f16b97
diff --git a/common.h b/common.h
index 319af3d..3e5f547 100644
--- a/common.h
+++ b/common.h
@@ -29,7 +29,7 @@
 extern bool modified_flash;
 // The current stage, e.g. "1/2".
-extern const char* stage;
+extern std::string stage;
 // The reason argument provided in "--reason=".
 extern const char* reason;