fix: do not set cmdline for uefi images unless asked

fix: do not set cmdline for uefi images unless asked

The kernel does not allow the cmdline to be overridden if one is defined
in the uefi image.  This means that bootloaders like systemd-boot or
grub cannot manage boot options (editing at boot for debug being a use
case).  This patchset allows for no cmdline to be set (instead of force
setting one if none is provided).

Fixes: https://github.com/dracutdevs/dracut/issues/1012
Signed-off-by: Matthew Thode <mthode@mthode.org>