export-image: when work in a non-English locale environment the PARTUUID will silently fail, resulting to an unbootable image file. (#96)

* use dd and xxd to get disk identifier

* add xxd as dependency

Dockerfile

@@ -4,7 +4,7 @@ RUN apt-get -y update && \
     apt-get -y install \
         git vim parted \
         quilt realpath qemu-user-static debootstrap zerofree pxz zip dosfstools \
-        bsdtar libcap2-bin rsync grep udev xz-utils curl \
+        bsdtar libcap2-bin rsync grep udev xz-utils curl xxd \
     && rm -rf /var/lib/apt/lists/*
 
 COPY . /pi-gen/