[U-Boot] [RFC] Migrate to Linux Kernel style SPDX Tags for v2018.05

Hey all,

This was already brought up by Heinrich Schuchardt, but didn't get much traction. So, I'm bringing it up again now. The little feedback from that thread was we should move to Linux Kernel style tags. I'm going to propose that we do just that, for v2018.05. I'll play with coccinelle to get it done. Comments? Thanks!