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/configs/kontron_sl28_defconfig b/configs/kontron_sl28_defconfig
index 1bae470..fcc6dd3 100644
--- a/configs/kontron_sl28_defconfig
+++ b/configs/kontron_sl28_defconfig
@@ -35,6 +35,7 @@
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_PCI_INIT_R=y
+CONFIG_SPL_MAX_SIZE=0x20000
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
 CONFIG_SPL_SPI_LOAD=y