mender-convert: specifying multiple device types causes mender-convert to fail

According to the documentation in mender_convert_config (see [here|https://github.com/mendersoftware/mender-convert/blob/master/configs/mender_convert_config#L57), multiple device types can be specified by providing a space-separated list of strings. This doesn't work unfortunately as the rest of the mender-convert code is not written to parse MENDER_DEVICE_TYPE as an array, but instead as a single string. This causes the build to fail.