spec: regenerate grub2 config after install

Apparently new-kernel-pkg does not work for grub2 config with xen
entries (new kernel isn't added to it at all). So regenerate grub2
config using grub2-mkconfig.

kernel.spec

@@ -364,6 +364,11 @@ fi
 %posttrans
 /sbin/new-kernel-pkg --package %{name}-%{kernelrelease} --rpmposttrans %{kernelrelease}
 
+# grubby (used by new-kernel-pkg) do not understand xen entries in grub2 config
+if [ -e /boot/grub2/grub.cfg ]; then
+        grub2-mkconfig > /boot/grub2/grub.cfg
+fi
+
 %preun
 /sbin/new-kernel-pkg --rminitrd --rmmoddep --remove %{kernelrelease}