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/at91sam9n12ek_spiflash_defconfig b/configs/at91sam9n12ek_spiflash_defconfig
index 8ba0658..17d90e6 100644
--- a/configs/at91sam9n12ek_spiflash_defconfig
+++ b/configs/at91sam9n12ek_spiflash_defconfig
@@ -5,6 +5,7 @@
 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="AT91SAM9N12,SYS_USE_SPIFLASH"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y