Improve cmake installation flags

[lrapetti]

This PR improves the usage of cmake flags for the installation of optional software components in wearables, trying to unify the behaviour.

In particular:

• avoids hidden if selecting software components
• change default flags values depending on the found dependencies

The documentation is also updated and improved accordingly

[lrapetti]

I forgot this PR behind. @traversaro @RiccardoGrieco if there are no further comments I would proceed with merging

[traversaro]

Ok for me.

[lrapetti]

Thanks!