refactor /boot-not-mounted detection to use util-linux tools

linux.install

@@ -44,12 +44,10 @@ post_upgrade() {
     echo ">>>"
   fi
 
-  if grep "^[^#]*[[:space:]]/boot" etc/fstab 2>&1 >/dev/null; then
-    if ! grep "[[:space:]]/boot" etc/mtab 2>&1 >/dev/null; then
+  if findmnt --fstab -uno SOURCE /boot &>/dev/null && ! mountpoint -q /boot; then
     echo "WARNING: /boot appears to be a separate partition but is not mounted."
     echo "         You probably just broke your system. Congratulations."
   fi
-  fi
 
   # updating module dependencies
   echo ">>> Updating module dependencies. Please wait ..."