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/stm32f469-discovery_defconfig b/configs/stm32f469-discovery_defconfig
index 7960618..4adb7e0 100644
--- a/configs/stm32f469-discovery_defconfig
+++ b/configs/stm32f469-discovery_defconfig
@@ -8,6 +8,7 @@
 CONFIG_STM32F4=y
 CONFIG_TARGET_STM32F469_DISCOVERY=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_SYS_LOAD_ADDR=0x400000
 CONFIG_BOOTDELAY=3
 # CONFIG_USE_BOOTCOMMAND is not set
 # CONFIG_DISPLAY_CPUINFO is not set