builddisks: Only install bootloader for new disk images
builddisks: Only install bootloader for new disk images

Assume the previous installation of syslinux succeeded.

vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,GNOME/gnome-ostree,chergert/gnome-continuous,GNOME/gnome-ostree,GNOME/gnome-ostree,chergert/gnome-continuous,chergert/gnome-continuous