olpc-os-builder config file format change

[Daniel Drake]

Hi,

I'm proposing the following change to olpc-os-builder configuration
files for 12.1.0 and onwards:

Currently, the set of modules to load is specified by a "modules"
configuration variable in the [global] section. Then, settings for
each module can be customised by adding a [section] for each module
and specifying the options there.

I propose that we simplify this: remove the "global.modules" variable,
and make the presence of a section become the hint that a module is
loaded. For example:

	[global]
	fedora_release=17
	modules=base,xo1,yumcfg

	[yumcfg]
	add_excludes_to=fedora-rawhide

Would become

	[global]
	fedora_release=17

	[base]
	[xo1]

	[yumcfg]
	add_excludes_to=fedora-rawhide



Reasoning: This is a simplification. Previously, many modules
effectively had to be listed twice - once in the module list, and
again to supply mandatory configuration.

All too often I have seen people (including myself) adding [sections]
to customise their build, but forgetting to add the module to the
list. And you don't realise your error until quite a while later.

Also, this simplifies the ability to pass multiple configuration files
to the build system. This will become more interesting as we move
towards adding build automation, where the build config file will be
the same everywhere, but those who want to bolt-on automatic build
publishing at the end will supply an additional config file (loading
futher modules) specifying addresses to push the build to, etc.

Thoughts/objections?

Thanks,
Daniel