Convert CONFIG_ENV_IS_IN_SPI_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_SPI_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/M54418TWR_serial_mii_defconfig b/configs/M54418TWR_serial_mii_defconfig
index 729acfe..47a786a 100644
--- a/configs/M54418TWR_serial_mii_defconfig
+++ b/configs/M54418TWR_serial_mii_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_TEXT_BASE=0x47E00000
 CONFIG_TARGET_M54418TWR=y
 CONFIG_SYS_EXTRA_OPTIONS="CF_SBF,SYS_SERIAL_BOOT,SYS_INPUT_CLKSRC=25000000"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="-> "