env: don't generate callback list entries for SPL

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>