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/at91rm9200ek_defconfig b/configs/at91rm9200ek_defconfig
index 0abfeab..b239fd9 100644
--- a/configs/at91rm9200ek_defconfig
+++ b/configs/at91rm9200ek_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_AT91RM9200EK=y
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=3
 # CONFIG_SYS_STDIO_DEREGISTER is not set
 # CONFIG_DISPLAY_CPUINFO is not set