Make "packages_file" really optional

Only try to install the packages from "package_file" if the file has been set
Make "pressed_file" really optional

Only set the preseesing for debconf if "preseed_file" has been set in config.ini
install "extra_packages_image" after preseeding and install the package list
clara images initrd: add a check to detect if 'packages_initrd' is empty
clara images initrd: add a check to detect if 'kver' is empty
do not create directory if the dirname is empty

If the dirname is empty, we save the image in our current directory ($PWD)
update changelog