qubes-kernel-vm-support.postinst run update-grub

https://github.com/QubesOS/qubes-issues/issues/5212
(cherry picked from commit 8821906186)

debian/qubes-kernel-vm-support.postinst

@@ -46,6 +46,19 @@ esac
 
 #DEBHELPER#
 
+## https://phabricator.whonix.org/T377
+## Debian has no update-grub trigger yet:
+## https://bugs.debian.org/481542
+
+if command -v update-grub >/dev/null 2>&1; then
+   update-grub || \
+      echo "$DPKG_MAINTSCRIPT_PACKAGE $DPKG_MAINTSCRIPT_NAME ERROR: Running \
+'update-grub' failed with exit code $?. $DPKG_MAINTSCRIPT_PACKAGE is most \
+likely only the trigger, not the cause. Unless you know this is not an issue, \
+you should fix running 'update-grub', otherwise your system might no longer \
+boot." >&2
+fi
+
 exit 0
 
 # vim: set ts=4 sw=4 sts=4 et :