Change bender target generation to come from the configuration files

[rgantonio]

This PR changes how to get bender targets and instead of declaring in Makefiles, we declare them in configuration files. This achieves the single source of truth.

This PR solves issue #294 as requested by @IveanEx

This matches with the Hemaia setup.

Major TODO:

• [x] Modify snaxgen.py by adding the bender targets utility.
• [x] Add bender targets on the configuration files.
• [x] Modify main Makefile to use this method.