Convert CONFIG_ENV_IS_IN_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/M5235EVB_Flash32_defconfig b/configs/M5235EVB_Flash32_defconfig
index 19596cb..ee8ba64 100644
--- a/configs/M5235EVB_Flash32_defconfig
+++ b/configs/M5235EVB_Flash32_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_TEXT_BASE=0xFFC00000
 CONFIG_TARGET_M5235EVB=y
 CONFIG_SYS_EXTRA_OPTIONS="NORFLASH_PS32BIT"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=1
 # CONFIG_DISPLAY_BOARDINFO is not set
 # CONFIG_CMD_LOADB is not set