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/da850evm_direct_nor_defconfig b/configs/da850evm_direct_nor_defconfig
index 99543d3..0a5c636 100644
--- a/configs/da850evm_direct_nor_defconfig
+++ b/configs/da850evm_direct_nor_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_DAVINCI=y
 CONFIG_TARGET_DA850EVM=y
 CONFIG_SYS_EXTRA_OPTIONS="MAC_ADDR_IN_SPIFLASH,USE_NOR,DIRECT_NOR_BOOT"
+CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_DISPLAY_CPUINFO is not set