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/nsa310s_defconfig b/configs/nsa310s_defconfig
index 8765dc3..4582b8d 100644
--- a/configs/nsa310s_defconfig
+++ b/configs/nsa310s_defconfig
@@ -9,6 +9,7 @@
 CONFIG_ENV_OFFSET=0xE0000
 CONFIG_DEFAULT_DEVICE_TREE="kirkwood-nsa310s"
 CONFIG_IDENT_STRING="\nZyXEL NSA310S/320S 1/2-Bay Power Media Server"
+CONFIG_SYS_LOAD_ADDR=0x800000
 CONFIG_BOOTDELAY=3
 CONFIG_USE_PREBOOT=y
 # CONFIG_DISPLAY_BOARDINFO is not set