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/ae350_rv32_spl_defconfig b/configs/ae350_rv32_spl_defconfig
index e01266b..cfd857c 100644
--- a/configs/ae350_rv32_spl_defconfig
+++ b/configs/ae350_rv32_spl_defconfig
@@ -18,6 +18,7 @@
 CONFIG_BOOTDELAY=3
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL_MAX_SIZE=0x100000
+CONFIG_SPL_BSS_START_ADDR=0x4000000
 CONFIG_SYS_PROMPT="RISC-V # "
 CONFIG_SYS_PBSIZE=1050
 CONFIG_CMD_IMLS=y