Use grub2-mkconfig if update-grub isn't present

update-grub doesn't exist on fedora so if the legacy grub config file is
being used use grub2-mkconfig instead.

Also slightly change the sed GRUB_CMDLINE_LINUX regex so that it will
match if the command line already contains entries.

Change-Id: I21ce382a0023d674a4a2e6afecc9bc678791eac9
Merge "Use grub2-mkconfig if update-grub isn't present"