Allow building arm images on x86 or arm hosts

As handy as cross-build-start is for building arm images on an x86
machine, it actually causes building to fail when on an arm machine.
Added an or statement to allow it to build on my rpi as well as my
laptop.

Dockerfile.raspbian

@@ -1,7 +1,7 @@
 FROM resin/rpi-raspbian:jessie
 MAINTAINER ViViDboarder <vividboarder@gmail.com>
 
-RUN [ "cross-build-start" ]
+RUN cross-build-start || true
 
 RUN apt-get update \
         && apt-get install -y --no-install-recommends \
@@ -22,7 +22,7 @@ RUN apt-get update \
             python-swiftclient \
         && rm -rf /var/apt/lists/*
 
-RUN [ "cross-build-end" ]
+RUN cross-build-end || true
 
 VOLUME "/root/.cache/duplicity"
 VOLUME "/backups"