Convert CONFIG_SPL_PAD_TO et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_PAD_TO
   CONFIG_SPL_MAX_SIZE
   CONFIG_TPL_PAD_TO
   CONFIG_TPL_MAX_SIZE

Note that we need to make TPL_MAX_SIZE be hex, and so move and convert the
existing places.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/scripts/Makefile.spl b/scripts/Makefile.spl
index f047d4e..c1d32f5 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -194,7 +194,7 @@
 
 # Turn various CONFIG symbols into IMAGE symbols for easy reuse of
 # the scripts between SPL, TPL and VPL.
-ifneq ($(CONFIG_$(SPL_TPL_)MAX_SIZE),)
+ifneq ($(CONFIG_$(SPL_TPL_)MAX_SIZE),0x0)
 LDPPFLAGS += -DIMAGE_MAX_SIZE=$(CONFIG_$(SPL_TPL_)MAX_SIZE)
 endif
 ifneq ($(CONFIG_$(SPL_TPL_)TEXT_BASE),)