ostree: Do not use grub2-mkconfig.

For more details see:
https://bugzilla.gnome.org/show_bug.cgi?id=762220

To summarize:

1) The current approach is less flexible than using a
   custom 'ostree-grub-generator' script. Each device can
   adjust this script for its needs, instead of hardcoding
   some values in 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.

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



Change-Id: I077838fdf16fbbb42f98df6fa6720bfbaf7c6f8b
Reviewed-by: Samuli Piippo <samuli.piippo@theqtcompany.com>