Improved dual core boot for amcfoc

[marcoaccame]

This PR improves the PR #481 and adds some performance tests on the the cores.

The boot strategy now uses a SystemInit() function, the one executed before the main(), that changes its behavior depending on the four cases of CORE_CM4 or CORE_CM7 when it is dualcore_BOOT_cm4master or dualcore_BOOT_cm7master.

I have also:

• cleaned the API of embot::hw::dualcore;
• added tentative configuration for debug using sync trace port w/ 4 bits;
• added two test functions that do floating point operations for comparison CM4 vs CM7.

This PR can be safely merged as it does not touch any other code than for board amcfoc.