rust-target-config: Create new class to contain target json config generation

rust-target-config: Create new class to contain target json config generation

Currently most of the rust recipes use this code but it is all piecemeal.
Turn the code into a class where things can start to be rationalised. Ultimately
some of the data and python code should be moved to a python library but one step
at a time.

No functionality changes.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>