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/mt7629_rfb_defconfig b/configs/mt7629_rfb_defconfig
index 43e7070..ea1a4ba 100644
--- a/configs/mt7629_rfb_defconfig
+++ b/configs/mt7629_rfb_defconfig
@@ -15,6 +15,7 @@
 CONFIG_SPL_PAYLOAD="u-boot-lzma.img"
 CONFIG_BUILD_TARGET="u-boot-mtk.bin"
 CONFIG_SPL_IMAGE="spl/u-boot-spl-mtk.bin"
+CONFIG_SYS_LOAD_ADDR=0x42007f1c
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_BOOTDELAY=3