[astro] initialize repo for astro release

Removes generic u-boot files and adds a README explaining how to build
the code. The next CLs will actually push the source.

Change-Id: Id120853f3d3e8360ba98029a0ffb7a6cead7bb5f
Reviewed-on: https://turquoise-internal-review.googlesource.com/c/third_party/u-boot/+/361771
Reviewed-by: Varun Sharma <vars@google.com>