pass startup_tcl to command_init

Removes external linkage from helper module, making the startup
code a parameter to a new command context's initialization routine.

src/openocd.h

@@ -36,4 +36,7 @@ void openocd_sleep_prelude(void);
 /// used by the server_loop() function in src/server/server.c
 void openocd_sleep_postlude(void);
 
+/// provides a hard-coded command environment setup
+extern const char *openocd_startup_tcl;
+
 #endif