Make the commands print a short usage summary and add a README file that
explains the dependencies.

Change-Id: I0c3f0713749ecfca23afaa2b536ac70dbdd7db0a
Reviewed-on: https://boringssl-review.googlesource.com/2471
Reviewed-by: Adam Langley <agl@google.com>

From upstream's 55f7fb8848.

Change-Id: I54ebc182addbf643bebc78aab03ba1327e24e2e7
Reviewed-on: https://boringssl-review.googlesource.com/2470
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)