[GSoC - x86_64] Minimal BSP to review

[Amaan Cheval]

Hi!

I've made a pull-request that's nearly complete on Github:
https://github.com/AmaanC/rtems-gsoc18/pull/1/

I'd appreciate a review before I squash it into 2 commits (1. minimal
BSP reaching Init task, and 2. adding the NS16550 console driver) and
submit the squashed patches to devel@ after this review.

I'll work on a blog post summarizing the work so far and the BSP
specific user documentation in the meantime.

Cheers,
Amaan