This module lets you tweak and customize the yum repos configuration in
the resultant build.

The tweaks you make here are not applied at build-time, they only apply
for yum usage once the image has been flashed onto an XO, the XO has
been booted, and the user runs 'yum'.

Note that OLPC doesn't really support yum usage, as it degrades various
internal systems (e.g. olpc-update) and workflows, and it is not robust
enough for field usage.

To add repositories, add uniquely-named options that start with "addrepo_",
one per repository, such as:

[yumcfg]
addrepo_1=1,olpc,http://mock.laptop.org/repos/local.10.1

There are 3 comma-separated fields in each addrepo option:
 1. whether or not packages in this repository contribute to the excludes list
    (0 or 1)
 2. The name of the repository
 3. The baseurl of the repository


If you mark a repository as contributing to the excludes list, the build
script will examine that repository and add all those packages to an
internal excludes list. Nothing happens with that excludes list, until
you use the add_excludes_to option:

[yumcfg]
addrepo_1=1,olpc,http://mock.laptop.org/repos/local.10.1
add_excludes_to=fedora,fedora-updates

The above example says that the "olpc" repository is to be added, but every
single package that exists in the "olpc" repository takes precedence over the
fedora and fedora-updates repositories, even if those repositories hold a
newer version.
(all packages in the olpc repo are excluded from fedora and fedora-updates)


Finally, there are 2 options which let you enable or disable the first
already-disabled or already-enabled repository in each repo file:

[yumcfg]
force_enable=updates-testing
force_disable=updates