Common branch for Cortex-M ports

[David Boddie]

The Inferno ports to Cortex-M4 and M7 CPUs shared a fair amount of code, so
I created a common directory for those ports to use, along the lines of the
sa1110 directory for related ports.

I created a new branch for this in my GitHub fork:

https://github.com/dboddie/inferno-os/tree/cortexm

I've also started to document how things in these ports work in this
repository:

https://github.com/dboddie/inferno-ports-doc

Hope this is useful to people!

David