spl: Change PAD_TO to CONFIG_SPL_PAD_TO

This was already used by some SPL targets, and allows the pad amount to
be specified by board config headers rather than only in makefile
fragments.

Also supply a pad-to of zero if the variable is undefined.  It works
without this, but this avoids relying on undocumented behavior.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
diff --git a/README b/README
index b5c1c03..5105e64 100644
--- a/README
+++ b/README
@@ -2836,6 +2836,10 @@
 		CONFIG_SPL_LIBGENERIC_SUPPORT
 		Support for lib/libgeneric.o in SPL binary
 
+		CONFIG_SPL_PAD_TO
+		Linker address to which the SPL should be padded before
+		appending the SPL payload.
+
 		CONFIG_SPL_TARGET
 		Final target image containing SPL and payload.  Some SPLs
 		use an arch-specific makefile fragment instead, for