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/M54451EVB_defconfig b/configs/M54451EVB_defconfig
index 63c1a4b..5f6382d 100644
--- a/configs/M54451EVB_defconfig
+++ b/configs/M54451EVB_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_TEXT_BASE=0x00000000
 CONFIG_TARGET_M54451EVB=y
 CONFIG_SYS_EXTRA_OPTIONS="SYS_INPUT_CLKSRC=24000000"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=1
 CONFIG_SYS_PROMPT="-> "
 # CONFIG_CMD_LOADB is not set