build: Find grub2-mkconfig a bit more automagically

build: Find grub2-mkconfig a bit more automagically

If one happens to not have grub2 installed, the previous commit
would cause us to fall back to `grub-mkconfig` with bad results.

We should likely just do dynamic detection and avoid pain, but
there's no harm in also allowing this to be statically determined.

The automagic here is that if `/etc/os-release` says `ID_LIKE=fedora`,
we know to use `grub2-mkconfig`.  But distro scripts should set this
in the build configurations using `--with-grub2-mkconfig=`.

Closes: #240
Approved by: jlebon