scripts/dts/extract_dts_includes: Add DT_ prefix to FLASH defines

We generate the following FLASH prefixed defines:
	FLASH_LABEL
	FLASH_ERASE_BLOCK_SIZE
	FLASH_WRITE_BLOCK_SIZE

Now we generate DT_ prefixed versions:
	DT_FLASH_LABEL
	DT_FLASH_ERASE_BLOCK_SIZE
	DT_FLASH_WRITE_BLOCK_SIZE

And will deprecate the non DT_ prefixed versions in the future.  We keep
them around as aliases for the time being.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>