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/M54455EVB_defconfig b/configs/M54455EVB_defconfig
index 557b87b..9c90ca5 100644
--- a/configs/M54455EVB_defconfig
+++ b/configs/M54455EVB_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_TEXT_BASE=0x04000000
 CONFIG_TARGET_M54455EVB=y
 CONFIG_SYS_EXTRA_OPTIONS="SYS_ATMEL_BOOT,SYS_INPUT_CLKSRC=33333333"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=1
 CONFIG_SYS_PROMPT="-> "
 CONFIG_CMD_IDE=y