Layer simplification

These commits try to make the layer more conformant to the Yocto Project guidelines and make it more usable for custom boards. The most notable changes are : 
  - Layer renaming to exclude the prefix "meta"
  - Create SoC files for boards that share the same MPU so have a lot of functionality in common
  - Rewrite at91bootstrap recipe to make it more generic and simplfy the recipe writing for a new version
  - Deploy at91bootstrap images to the deploy directory, ready for flashing