bootloader/grub2: Don't do anything if we have static configs

bootloader/grub2: Don't do anything if we have static configs

This builds on top of https://github.com/coreos/bootupd/pull/609/commits/fa9924e4fe403c3751392c041cd98614a2cc3611
(But in a very hacky way because we don't currently link to a JSON library)

Basically, bootupd supports injecting static configs, and this
is the currently least hacky way for us to detect this and understand
that we shouldn't try to run `grub2-mkconfig`.

A further patch I'd like to do here is also change the probing
logic to gracefully no-op if `grub2-mkconfig` doesn't exist,
but that has a bit more risk and involvement.