Convert CONFIG_SYS_LOAD_ADDR to Kconfig

Now that we have consistent usage, migrate this symbol to Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/configs/thuban_defconfig b/configs/thuban_defconfig
index 43b7830..9d85796 100644
--- a/configs/thuban_defconfig
+++ b/configs/thuban_defconfig
@@ -22,6 +22,7 @@
 CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI_SUPPORT=y
 CONFIG_ENV_VARS_UBOOT_CONFIG=y
+CONFIG_SYS_LOAD_ADDR=0x81000000
 CONFIG_BOOTDELAY=3
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Autobooting in %d seconds, press \"<Esc><Esc>\" to stop\n"