Convert CONFIG_SPL_BSS_START_ADDR to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_BSS_START_ADDR

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/nanopi-r4s-rk3399_defconfig b/configs/nanopi-r4s-rk3399_defconfig
index ca10940..917fd3f 100644
--- a/configs/nanopi-r4s-rk3399_defconfig
+++ b/configs/nanopi-r4s-rk3399_defconfig
@@ -18,6 +18,8 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_SPL_MAX_SIZE=0x2e000
 CONFIG_SPL_PAD_TO=0x7f8000
+CONFIG_SPL_HAS_BSS_LINKER_SECTION=y
+CONFIG_SPL_BSS_START_ADDR=0x400000
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set