This commit fixes all copyright notes, using "git blame" as a
reference.  For simple files I just added a 2-lines boilerplate about
being WR and GPL.

Some parts I left untouched:

  trivial-init and related files (unclear to me what this is,
  and pf-microcode.h is magic binary pfilter values).

  tools/lm32-etheruart: I don't know what this is and who the author is

  libsdbfs: this comes from another repo and I'd better not change it

  sfp_lib.h and hal_shmem.h: I lost track about the origin of this,
  among the several packages.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>