Move adb_main to its own file.

Change-Id: If6e98c089b39f73bd8f4a878a82669bfeee367f3
diff --git a/adb/adb_auth.h b/adb/adb_auth.h
index 4431b5b..cece5e3 100644
--- a/adb/adb_auth.h
+++ b/adb/adb_auth.h
@@ -21,6 +21,8 @@
 extern "C" {
 #endif
 
+extern int auth_enabled;
+
 void adb_auth_init(void);
 int adb_auth_keygen(const char* filename);
 void adb_auth_verified(atransport *t);