Support multiple C toolsets in same build (e.g. SPU and PPU) about tundra

[deplinenoise]

This is very similar to #20, but with a twist.

[jsvennevid]

+1 on this, since we'll mix the use of EE and IOP toolchains in the future. :)

[deplinenoise]

Here's one way to implement this that seems clean:

• Have configs in tundra.lua list SubConfigs that they can need to build stuff with. This covers both cross-compilation (need to use a toolset compatible with the host) and SPU/PPU case (just two different C compilers really)
• In boot.lua, create base environment and load tools for all subconfigs too
• When generating nodes, decide on what base environment to use depending on the requirements of the program. This can be done either with plain filtering or with some other tagging mechanism.

[deplinenoise]

Cross-compilation and multiple toolsets per config.

• Added SubConfigs concept that lets a configuration specify that it is the union of several other configurations.
• Added Virtual property to configuration to flag that they cannot be built directly. For example, for a PS3 setup you would need three nested configs, a host config to compile tools with, a PPU config and an SPU config. These three would be marked virtual.
• Added HOSTPROGSUFFIX, HOSTSHLIBSUFFIX to be used in cross-compilation scenarios when running tools.

Closed by 277d6ca (Support multiple C toolsets in same build (e.g. SPU and PPU))
Closed by 277d6ca (Support for cross-compilation)