Convert CONFIG_SPL_SPI_FLASH_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/cm_t43_defconfig b/configs/cm_t43_defconfig
index 81dd073..5779f17 100644
--- a/configs/cm_t43_defconfig
+++ b/configs/cm_t43_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
+CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_MTD_SUPPORT=y