Better dynamic linker

The current dynamic linker, implemented in-kernel for userspace processes, is more a proof-of-concept than a very usable and powerful design. We'll have to re-think it, and to determine if a userspace dynamic linker isn't a better solution (its the one chosen by many mature OS).

In all cases, the current implementation is flawed because of :

• usage of external tools to create the FiXos-specific sections used to retrieve symbols and their addresses
• no way to provide an external variable from another (shared) object, only functions are linkable
• only one dynamic library by executable, and loaded at the same address each time (not a big deal to change that, but it's a current issue)