buildroot

lightning: fix build issue by bumping to 2.1.2

The current version of lightning doesn't build with recent versions of
binutils. Since this has been fixed upstream in a dot release, let's
bump to the upstream version that has the fix.

While at it, add a hash for the license file.

Fixes:

  http://autobuild.buildroot.net/results/51424c044e11b89544824dbdf1af45b05b499ba4/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>