Generate platfrom.zip for build goal "platform".

Bug: 6241618
If platform appears on the build command line, build platform.zip,
which essentially is the zip package of the system directory.
Put the platform.zip to the dist dir if "dist" is specified too,
so the platform.zip can be downloaded from the build page.

Change-Id: I988dd5c753a4ee744b4e3dccf261d8df904d9847