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/sifive_unleashed_defconfig b/configs/sifive_unleashed_defconfig
index 326bca7..10c6934 100644
--- a/configs/sifive_unleashed_defconfig
+++ b/configs/sifive_unleashed_defconfig
@@ -25,6 +25,7 @@
 CONFIG_DISPLAY_BOARDINFO=y
 CONFIG_MISC_INIT_R=y
 CONFIG_SPL_MAX_SIZE=0x100000
+CONFIG_SPL_BSS_START_ADDR=0x85000000
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
 CONFIG_SPL_STACK=0x81cfe70
 CONFIG_SPL_DM_SPI_FLASH=y