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/silk_defconfig b/configs/silk_defconfig
index 0652412..6cbe5aa 100644
--- a/configs/silk_defconfig
+++ b/configs/silk_defconfig
@@ -28,6 +28,7 @@
 CONFIG_ENV_ADDR=0xC0000
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
+CONFIG_SPL_MAX_SIZE=0x4000
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_RAM_SUPPORT=y