util/grub-install: Delay copying files to {grubdir,platdir} after install_device...

util/grub-install: Delay copying files to {grubdir,platdir} after install_device was validated

Previously grub-install copied modules to grubdir before doing any
validation on the install_device.

When grub-install was called with an invalid install_device, modules
were already copied to /boot before it found out and was forced to rely
on atexit() rollback.

This patch delays copying the modules after at least some install_device
validation was done, and thus reduces reliance on successful rollback.

Signed-off-by: Mate Kukri <mate.kukri@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>