Introducing ostree-grub-generator

Introducing ostree-grub-generator

ostree-grub-generator can be used to customize
the generated grub.cfg file. Compile time
decision ostree-grub-generator vs grub2-mkconfig
can be overwritten with the OSTREE_GRUB2_EXEC
envvar - useful for auto tests and OS installers.

Why this alternative approach:

1) The current approach is less flexible than using a
   custom 'ostree-grub-generator' script. Each system can
   adjust this script for its needs, instead of using the
   hardcoded values from ostree-bootloader-grub2.c.

2) Too much overhead on embedded to generate grub.cfg
   via /etc/grub.d/ configuration files. It is still
   possible to do so, even with this patch applied.
   No need to install grub2 package on a target device.

3) The grub2-mkconfig code path has other issues:
   https://bugzilla.gnome.org/show_bug.cgi?id=761180

Task: https://bugzilla.gnome.org/show_bug.cgi?id=762220

Closes: #228
Approved by: cgwalters