Use grub package instead of grub-pc unless distro is ubuntu

According to the Debian wiki, installing the package grub installs the
correct version of grub for your suite automatically. This fixes a "you
have held broken packages" error when creating the package manifest due
to a conflict between grub-pc and grub-legacy (at least with Wheezy).

bin/make-base-vm

@@ -122,11 +122,16 @@ if [ $DISTRO = "ubuntu" ]; then
   KERNEL_PKG=linux-image-generic,
 fi
 
+GRUB_PKG=grub
+if [ $DISTRO = "ubuntu" ]; then
+  GRUB_PKG=grub-pc
+fi
+
 if [ $LXC = "1" ]; then
   addpkg=$addpkg,lxc
 else
   # Lack of comma after KERNEL_PKG is not a typo
-  addpkg=$addpkg,${KERNEL_PKG}grub-pc,openssh-server
+  addpkg=$addpkg,${KERNEL_PKG}${GRUB_PKG},openssh-server
 fi
 
 # Remove cron to work around vmbuilder issue when umounting /dev on target