Make Linux packaging options more granular.

Make Linux packaging options more granular.
    
This allows us to not waste time building packages/channels we don't care about. Also added option to build new 'trunk' channel packages, which are a special configuration for developer testing.

[Lei Zhang, 2009-11-20 22:48:22]

LGTM

[Michael Moss, 2009-11-20 23:09:15]

I'm rescinding this change. After talking to laforge, we figure the tedium of
updating this variable when we branch builds (assuming we don't have dedicated
builders for each channel to set the override properly) outweighs the benefits
of limiting the number of packages built. I'll send a new CL to implement this
as an optional, rather than the default, behaviour.

[Michael Moss, 2009-11-23 19:12:35]

Updated to retain existing default behavior, with option to specify channel for
restricted package building.

[Lei Zhang (Do not use), 2009-11-23 19:37:56]

LGTM

The lack of a gyp looping construct makes me want to cry.