package/cache-calibrator: fix legal-info

package/cache-calibrator: fix legal-info

Fixes:
http://autobuild.buildroot.net/results/e1b/e1b3dee917f5ab3961f3dea006720431444d0ef5/

cache-calibrator is a single C file that is downloaded as-is, and we
patch it. We also use that file as the license file.

Thus, the file when it is downloaded has a specific hash, but when
saved by legal-info, it has a different hash.

However, we can't store two different hashes for the same _filename_
with different content.

So, we extract the license file as a pre-patch hook, and store it in a
separate file, for which we can now add a hash.

[Peter: add autobuilder reference]
Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Stephan Hoffmann <sho@relinux.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>