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/imx7_cm_defconfig b/configs/imx7_cm_defconfig
index 351d47f..1fc16f5 100644
--- a/configs/imx7_cm_defconfig
+++ b/configs/imx7_cm_defconfig
@@ -25,6 +25,7 @@
 CONFIG_DEFAULT_FDT_FILE="ask"
 # CONFIG_BOARD_EARLY_INIT_F is not set
 CONFIG_SPL_MAX_SIZE=0xe000
+CONFIG_SPL_HAS_BSS_LINKER_SECTION=y
 CONFIG_SPL_BSS_MAX_SIZE=0x100000
 CONFIG_SPL_I2C=y
 CONFIG_SPL_USB_HOST=y