Licenses/README: Update some style and add explicit license to the document

- Add an SPDX license tag to the file, saying it's GPL-2.0.
- From the Linux Kernel v4.17-rc4, import the "License identifier
  syntax" section as-is from Documentation/process/license-rules.rst
  and then change it to be clearer about examples from the Linux Kernel
  vs examples found in U-Boot, and when we're talking about U-Boot.

Signed-off-by: Tom Rini <trini@konsulko.com>