Detect and prevent multiple providers (branch yem/fix-virt-pkg)

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"Yann" == Yann E MORIN <[email protected]> writes:

From: "Yann E. MORIN" <[email protected]>
Hello All!

Following a weird report about a cairo build failure, we came to the conclusion that the problem was due to multiple providers of the same virtual package were selected in the failing configuration. See the thread starting there:

    http://lists.busybox.net/pipermail/buildroot/2014-May/096379.html

The matter was discussed on the list, and we came up to the conclusion we would not implement anything in the menuconfig to prevent this situation, but that we would just detect it prior to running the build. See the thread starting there:

    http://lists.busybox.net/pipermail/buildroot/2014-May/096517.html

So, what this series does is just that: detect that more than one provider for a virtual package are selected.

The way it does that is fully described in patch 1/3, but basically:

- providers must now declare the virtual packages they are an

    implementation of, in their .mk file;

- the generic package infrastructure checks that for each such declared virtual package, the current provider is indeed the selected implmenetation. If not, then it means two or more providers for that virtual package are selected, and it bails out very early in the build.

Changes v1 -> v2:
- fix providers for luainterpreter (Thomas P.)