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/T1042D4RDB_defconfig b/configs/T1042D4RDB_defconfig
index 410a124..8daff4d 100644
--- a/configs/T1042D4RDB_defconfig
+++ b/configs/T1042D4RDB_defconfig
@@ -6,6 +6,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=10
 CONFIG_SILENT_CONSOLE=y
 # CONFIG_CONSOLE_MUX is not set