[legal] Add UBDL relicensing tool

[legal] Add UBDL relicensing tool

The UBDL relicensing tool (util/relicense.pl) is designed to identify
files which may be relicensed under a dual GPL+UBDL licence.  It uses
git-blame to identify the author of each line (using the -M and -C
options to track lines moved or copied between files), and relicenses
files for which all authors have given permission.

The relicensing tool will ignore certain types of lines identified by
git-blame:

 - empty lines
 - comments
 - standalone opening or closing braces
 - "#include ..."
 - "return 0;"
 - "return rc;"
 - "PCI_ROM(...)"
 - "FILE_LICENCE(...)"

These lines either contain no meaningful content (e.g. empty lines),
contain only non-copyrightable facts (e.g. PCI ROM IDs) or are
sufficiently common within the codebase that git-blame is likely to
misattribute their origin (e.g. "return 0").

Signed-off-by: Michael Brown <mcb30@ipxe.org>