Rename PLATFORM and FLAVOUR

[arnout]

"linux" platform is actually a "posix" platform (cfr. it also works on Cygwin).

"x86_generic" flavour is actually "generic", not at all tied to x86.

Instead of flavours, we probably want to include/exclude plugins (or extensions) for different platforms, and the generic part is always there. We can still call it flavour, but it should be possible to support multiple. E.g. the simulator could be optional.