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/mt7628_rfb_defconfig b/configs/mt7628_rfb_defconfig
index 4684873..7690213 100644
--- a/configs/mt7628_rfb_defconfig
+++ b/configs/mt7628_rfb_defconfig
@@ -24,6 +24,7 @@
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_SYS_MALLOC_BOOTPARAMS=y
 CONFIG_SPL_MAX_SIZE=0x10000
+CONFIG_SPL_BSS_START_ADDR=0x80010000
 CONFIG_SPL_BSS_MAX_SIZE=0x10000
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_NOR_SUPPORT=y