Ftp hosting

Here is some tweaks to host simulator xpis to a ftp.
I'm not sure the `release` target has its place in this Makefile, but that ends up being very handy and ensure that we upload xpis to the precise right place. But may be we can move it to ftp folder... or keep it private.

I tested by uploading a first pre3 version with its update.rdf.
No update requested when installing it.
Then uploaded pre4 based on the second revision, and also uploaded its related update.rdf,
the pre3 immediatly request an update and the update works. (I checked addon version, but also b2g version in the settings app)