util/grub-install: Set point of no return for powerpc-ieee1275 install

util/grub-install: Set point of no return for powerpc-ieee1275 install

The point of no return is used to define a point where no change should
be reverted in a wake of fatal error that consequently aborts the
process. The powerpc-ieee1275 install apparently missed this point of no
return definition that newly installed modules could be inadvertently
reverted after successful image embedding so that boot failure is
incurred due to inconsistent state.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>