grub: make boot RAID check Petitboot-compatible
Petitboot doesn't support -e and doesn't support testing for a bare
device without a path component.
set pager=1
-if [ -e (md/md-boot) ]; then
+# petitboot doesn't support -e and doesn't support an empty path part
+if [ -d (md/md-boot)/grub2 ]; then
# fcct currently creates /boot RAID with superblock 1.0, which allows
# component partitions to be read directly as filesystems. This is
# necessary because transposefs doesn't yet rerun grub2-install on BIOS,
