env: don't generate callback list entries for SPL

SPL doesn't write to the environment.  These list entries prevent the
functions from being garbage-collected, even though nothing will look at
the list.  This caused several SPL builds (e.g.  P2020RDB-PC_NAND) to
break due to size limitations and/or unresolved symbols.

A static inline function is used to provide a context in which we
can consume the callback, and thus avoid unused function warnings.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Joe Hershberger <joe.hershberger@gmail.com>
Acked-by: Kim Phillips <kim.phillips@freescale.com>

include/env_callback.h

@@ -68,8 +68,16 @@ void env_callback_init(ENTRY *var_entry);
  * when associated through the ".callbacks" environment variable, the callback
  * will be executed any time the variable is inserted, overwritten, or deleted.
  */
+#ifdef CONFIG_SPL_BUILD
+#define U_BOOT_ENV_CALLBACK(name, callback) \
+	static inline void _u_boot_env_noop_##name(void) \
+	{ \
+		(void)callback; \
+	}
+#else
 #define U_BOOT_ENV_CALLBACK(name, callback) \
 	ll_entry_declare(struct env_clbk_tbl, name, env_clbk, env_clbk) = \
 	{#name, callback}
+#endif
 
 #endif /* __ENV_CALLBACK_H__ */