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/socrates_defconfig b/configs/socrates_defconfig
index 5def6d5..f0578a5 100644
--- a/configs/socrates_defconfig
+++ b/configs/socrates_defconfig
@@ -6,6 +6,7 @@
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=1
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y