Preserve Linux capabilities when running tar, and misc fixes for mkrootfs. (#92)

* Preserve Linux capabilities in various tar invocations.

Otherwise, ping as non-root will not work when using a Void image.

Don't try to umount pseudofilesystems when $rootfs is undefined.
On mktemp failure, die in the script itself, not the $( ... ) subshell.
And remove the second FATAL: from the error message.

mkimage.sh.in

@@ -179,7 +179,7 @@ if [ "$PLATFORM" = "beaglebone" ]; then
     touch $ROOTFSDIR/boot/uEnv.txt
     umount $ROOTFSDIR/boot
 fi
-tar xfp $ROOTFS_TARBALL -C $ROOTFSDIR
+tar xfp $ROOTFS_TARBALL --xattrs --xattrs-include='*' -C $ROOTFSDIR
 
 fspassno="1"
 if [ "$ROOT_FSTYPE" = "f2fs" ]; then