i2c-tools: include LGPL-2.1+ license for libi2c

Extend i2c-tools SPDX identifiers to include the library license.
Also include COPYING.LGPL and README to license files.

The ic2-tools readme states:

LICENSE

Check the documentation of individual tools for licensing information.
The library is released under the LGPL version 2.1 or later, while most
tools are released under the GPL version 2 or later, but there are a few
exceptions.

Signed-off-by: Brad Love <brad@nextdimension.cc>
[Thomas: add hashes for COPYING.LGPL and README.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>