Add license/fastlog2.txt

Add license file (BSD license) for fastlog2() function used in the
new scheduler as a quick approximate logf().
From https://code.google.com/p/fastapprox/.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>