cmd: rework "license" command

The previous commit ("add a new command to show .config contents")
improves the basic infrastructure of "embed a compressed file into
the U-Boot image, and print it by a command".  The same pattern for
the "license" command.

This commit reworks the command to improve the following:

[1] Improve log style

Kbuild style log

  GZIP    cmd/license_data.gz
  CHK     cmd/license_data_gz.h
  UPD     cmd/license_data_gz.h
  CHK     cmd/license_data_size.h
  UPD     cmd/license_data_size.h

instead of the bare Make log:

cat ./Licenses/gpl-2.0.txt | gzip -9 -c | \
		tools/bin2header license_gzip > ./include/license.h

[2] Collect related code into the "cmd" directory

Prior to this commit, the license.h was created by tools/Makefile,
placed under the "include" directory, included from cmd/license.c,
and deleted by the top-level Makefile.  It is not a good idea to
scatter related code.

[3] Drop the fixed-malloc size LICENSE_MAX

Just allocate the minimum required size of buffer because we know
the size of the original gpl-2.0.txt.

[4] Fix more issues

Terminate the buffer with zero to prevent puts() from over-running.
Add "static" to do_license.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>