Add system-update script

The `system-update` script takes an archive package (`metricspi.tar`)
and extracts its contents into the proper locations to update the
system.

rootfs/overlay/usr/sbin/system-update

@@ -0,0 +1,76 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+check_system() {
+    rc=0
+    if ! mountpoint -q /boot; then
+        echo 'ERROR: /boot is not mounted' >&2
+        rc=1
+    elif [ ! -w /boot ]; then
+        echo 'ERROR: /boot is not writable' >&2
+        rc=1
+    fi
+    if ! mountpoint -q /boot/efi; then
+        echo 'ERROR: /boot/efi is not mounted' >&2
+        rc=1
+    elif [ ! -w /boot/efi ]; then
+        echo 'ERROR: /boot/efi is not writable' >&2
+        rc=1
+    fi
+    return ${rc}
+}
+
+die() {
+    rc=$?
+    echo "$1" >&2
+    exit ${rc}
+}
+
+do_update() {
+    if [ -d /boot/efi/.old ]; then
+        rm -rf /boot/efi/.old || die 'Failed to remove /boot/efi/.old'
+    fi
+    mkdir /boot/efi/.old || die 'Failed to create /boot/efi/.old'
+    for x in /boot/efi/*; do
+        mv "${x}" /boot/efi/.old/ \
+            || die "Failed to move ${x} to /boot/efi/.old/"
+    done
+    mv /boot/rootfs.squashfs /boot/rootfs.squashfs.old \
+        || die 'Failed to rename rootfs.squashfs'
+    echo 'Extracting root filesystem ...'
+    tar -xf "${filename}" -C /boot rootfs.squashfs \
+        || die 'Failed to extract rootfs.squashfs'
+    echo 'Extracting kernel/initramfs/firmware ...'
+    tar -xf "${filename}" -C /boot/efi \
+        --exclude rootfs.squashfs \
+        --no-same-owner \
+        || die 'Failed to extract kernel/initramfs/firmware'
+    echo 'The machine will need to be rebooted to complete the update process'
+}
+
+usage() {
+    printf 'usage: %s filename\n' "${0##*/}"
+}
+
+filename=
+
+while [ $# -ge 1 ]; do
+    case "$1" in
+        -h|--help)
+            usage
+            exit 0
+            ;;
+        *)
+            filename="$1"
+            shift
+            ;;
+    esac
+done
+
+if [ -z "${filename}" ]; then
+    usage >&2
+    exit 2
+fi
+
+check_system || exit $?
+do_update