Check GRUB presence via kenv instead of fstab
It seems after #32412 FreeNAS 11.2 installs empty fstab, overwriting one that has GRUB line in it, that blocks `grep -q "grub" /conf/base/etc/fstab` in ix-bsdloader. As result, on WebUI update GRUB remains as loader, but probably won't function correctly, since we started to remove other required parts.
- Status changed from Unscreened to In Progress
Unless there are better ideas, as possible solution I propose to replace `grep -q "grub" /conf/base/etc/fstab` check there, used to detect GRUB presence, with `kenv -q grub.platform >/dev/null`. Any objections?