Generate platfrom.zip for build goal "platform".
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

knone1/platform_manifest,fallouttester/platform_manifest,xorware/android_build,RR-msm7x30/platform_manifest,xorware/android_build,xorware/android_build,hagar006/platform_manifest,xorware/android_build,ND-3500/platform_manifest,xorware/android_build,pranav01/platform_manifest,see4ri/lolili,Hybrid-Power/startsync