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/firefly-px30_defconfig b/configs/firefly-px30_defconfig
index 31374c5..bef490d 100644
--- a/configs/firefly-px30_defconfig
+++ b/configs/firefly-px30_defconfig
@@ -33,6 +33,8 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_SPL_MAX_SIZE=0x20000
 CONFIG_SPL_PAD_TO=0x7f8000
+CONFIG_SPL_HAS_BSS_LINKER_SECTION=y
+CONFIG_SPL_BSS_START_ADDR=0x4000000
 CONFIG_SPL_BSS_MAX_SIZE=0x4000
 CONFIG_SPL_BOOTROM_SUPPORT=y
 # CONFIG_SPL_RAW_IMAGE_SUPPORT is not set