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/porter_defconfig b/configs/porter_defconfig
index a50d9ed..d74a954 100644
--- a/configs/porter_defconfig
+++ b/configs/porter_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_RMOBILE=y
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_TARGET_PORTER=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 CONFIG_CMD_BOOTZ=y