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/kmnusa_defconfig b/configs/kmnusa_defconfig
index 635ecb1..57aa3a8 100644
--- a/configs/kmnusa_defconfig
+++ b/configs/kmnusa_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_KM_KIRKWOOD=y
 CONFIG_IDENT_STRING="\nKeymile NUSA"
 CONFIG_SYS_EXTRA_OPTIONS="KM_NUSA"
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y