Definition for managing Grub 2 config options.

This adds a definition bootloader::grub2::option which can be used
to manage options in /etc/default/grub.  These options are used by
the scripts in /etc/grub.d to control their output when generating
/boot/grub2/grub.cfg.

As these options are so different from the ones Grub 0.x uses, there
is no meaningful way to have a common abstracting definition that just
dispatches to the correct provider; instead this definition, and the
somewhat corresponding bootloader::grub0::option, are intended to be
used directly by users.