Rename legacy nRF51822 targets, and add boards using new common target

23 Jan 2018

I'm working on a pull request to clean up the targets file and make it better conform with the style guide, regarding redundent labels, using config instead of macros, etc. Im fairly sure the nRF51822 HAL platforms are now considered legacy, should they be renamed to something like `MCU_NRF51_LEGACY ` to make it clear that the new unified platform should be used instead?