makedevs: add license information

[Thomas: change license to be GPLv2. The header of makedevs.c is a bit
unclear, as it mentions both the GPL and LGPL. But since the code
seems to originally be coming from Busybox, assuming GPLv2 is the
safest option.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>