Include in mar names, because they're uploaded to common directories.

As part of switching over B2G updates to our production-grade update server we'll end up uploading MARs to common directories like https://ftp.mozilla.org/pub/mozilla.org/b2g/nightly/2014-04-25-04-02-03-mozilla-central/. Because we'll be doing this for the gecko+gaia bits on multiple different devices, we need to make sure the files don't stomp on each other. This patch should do that. I tested it by hand to make sure that $(PRODUCT_MODEL) was available in this Makefile.

Relevant bugs:
https://bugzilla.mozilla.org/show_bug.cgi?id=1000207
https://bugzilla.mozilla.org/show_bug.cgi?id=918068