Support for cross-compilation

[deplinenoise]

Tundra can already build foreign programs just fine, but a cross-compilation aware configuration is needed e.g. for consoles when compiling tools.

[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)