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/P2020RDB-PC_SPIFLASH_defconfig b/configs/P2020RDB-PC_SPIFLASH_defconfig
index f6d5fc9..64fc8e6 100644
--- a/configs/P2020RDB-PC_SPIFLASH_defconfig
+++ b/configs/P2020RDB-PC_SPIFLASH_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
+CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_MPC85xx=y
 CONFIG_TARGET_P1_P2_RDB_PC=y
 CONFIG_FIT=y