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/devkit8000_defconfig b/configs/devkit8000_defconfig
index 9bc9c0c..9ab5423 100644
--- a/configs/devkit8000_defconfig
+++ b/configs/devkit8000_defconfig
@@ -13,6 +13,7 @@
 CONFIG_BOOTCOMMAND="run autoboot"
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_SPL_MAX_SIZE=0xec00
+CONFIG_SPL_BSS_START_ADDR=0x80000500
 CONFIG_SPL_NAND_DRIVERS=y
 CONFIG_SPL_NAND_ECC=y
 CONFIG_SPL_NAND_SIMPLE=y