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/dreamplug_defconfig b/configs/dreamplug_defconfig
index 8f3ffcf..640096b 100644
--- a/configs/dreamplug_defconfig
+++ b/configs/dreamplug_defconfig
@@ -2,6 +2,7 @@
 CONFIG_KIRKWOOD=y
 CONFIG_TARGET_DREAMPLUG=y
 CONFIG_IDENT_STRING="\nMarvell-DreamPlug"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y