package/devmem2: fix legal-info

package/devmem2: fix legal-info

Fixes:
http://autobuild.buildroot.net/results/1c6/1c69c77d7b378f6f76790fb5ca0078d0db0c40f8

devmem2 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.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>