dt: new package

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
[Thomas: use <pkg>_STRIP_COMPONENTS = 2, which allows to get rid of
the <pkg>_SUBDIR usage, and make things a bit simpler.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>