move grub-pc specific code to actual grub-pc script
reintroduces the commit that was dropped from a different MR due to my having forgotten to add an ISO based condition around it in the new location, with this now fixed of course.
the grub-pc image creation code has no business being in binary_iso, it should be in binary_grub-pc.
it should be noted that the binary_iso script did not even have the necessary package check for grub-mkimage, while binary_grub-pc did have it, pointlessly.
Gbp-Dch: Short