caches: find a more elegant way to activate/deactivate cache support

[jnaulet]

Right now there are ugly ifdefs related to caches in picoRTOS' code cause some CPUs have caches, some don't and it's hard to find the right spot to manage this elegantly.

Here are some leads:

• Make all archs support piocRTOS' cache API, even if functions are empty (make ports slighly less convenient)
• Put ifdefs everywhere (ugly & probably not compliant with the JPL rules)
• Something else ?

[jnaulet]

Here's an experimental attempt at solving this issue aa83824