Do not clear variable in the initramfs hook, if already set.

In the hook, when initializing variables do not clear their pre-existing values, which may have been set in via conf.d hooks-conf.d directories in /etc and /usr as a way to configure initramfs generation.

It is useful to change the initramfs hook default paremeters via drop-in directories like all the other initrmafs hooks, without modifying the conffile in /etc/defaults reserved for the end administrators.