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/kontron_pitx_imx8m_defconfig b/configs/kontron_pitx_imx8m_defconfig
index 4660e98..e8f653b 100644
--- a/configs/kontron_pitx_imx8m_defconfig
+++ b/configs/kontron_pitx_imx8m_defconfig
@@ -31,6 +31,8 @@
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_MISC_INIT_R=y
 CONFIG_SPL_MAX_SIZE=0x1f000
+CONFIG_SPL_HAS_BSS_LINKER_SECTION=y
+CONFIG_SPL_BSS_START_ADDR=0x180000
 CONFIG_SPL_BSS_MAX_SIZE=0x2000
 # CONFIG_SPL_SHARES_INIT_SP_ADDR is not set
 CONFIG_SPL_STACK=0x187ff0