prevent upgrade of grub-pc which can fail in lxc

fixes #86

target-bin/grab-packages.sh

@@ -7,6 +7,7 @@ set -e
 cd /var/cache/apt/archives
 
 # make sure all packages with installed versions are downloaded
-dpkg-query -W -f '${Package}=${Version}\n' | xargs -n 50 apt-get install -q --reinstall -y -d > /tmp/download.log
+# (except for held packages, which may not be available for download)
+dpkg-query -W -f '${Status}\t${Package}=${Version}\n' | grep -v ^hold | cut -f2- | xargs -n 50 apt-get install -q --reinstall -y -d > /tmp/download.log
 grep "cannot be downloaded" /tmp/download.log && { echo Could not download some packages, please run gbuild --upgrade 1>&2 ; exit 1 ; }
 sha256sum *.deb | sort --key 2

target-bin/upgrade-system.sh

@@ -9,6 +9,9 @@ mkdir -p /var/cache/gitian
 # remove obsolete grub, it causes package dependency issues
 apt-get -q -y purge grub > /dev/null 2>&1 || true
 
+# prevent upgrade of grub-pc, it fails to find a boot drive in lxc containers
+echo grub-pc hold | dpkg --set-selections || true
+
 # upgrade packages
 DEBIAN_FRONTEND=noninteractive apt-get -y dist-upgrade > /dev/null > /var/cache/gitian/upgrade.log 2>&1