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/ci20_mmc_defconfig b/configs/ci20_mmc_defconfig
index a1c2fa7..983642d 100644
--- a/configs/ci20_mmc_defconfig
+++ b/configs/ci20_mmc_defconfig
@@ -24,6 +24,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_SYS_MALLOC_BOOTPARAMS=y
 CONFIG_SPL_MAX_SIZE=0x2e00
+CONFIG_SPL_BSS_START_ADDR=0xf4004000
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
 CONFIG_SPL_STACK=0xf4008000