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/da850_am18xxevm_defconfig b/configs/da850_am18xxevm_defconfig
index 69a47b9..8229f1a 100644
--- a/configs/da850_am18xxevm_defconfig
+++ b/configs/da850_am18xxevm_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
+CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="DA850_AM18X_EVM,MAC_ADDR_IN_EEPROM,SYS_I2C_EEPROM_ADDR_LEN=2,SYS_I2C_EEPROM_ADDR=0x50"
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y