Don't overwrite vars that start with GRUB_CMDLINE_LINUX
When updating args for ALL kernels, grubby clobbers all variables
in /etc/default/grub that start with GRUB_CMDLINE_LINUX and renders
that line multiple times, for each variable that exists. This breaks
using recovery mode. Fix so this doesn't happen anymore.
Signed-off-by: Marta Lewandowska <mlewando@redhat.com>