dtc: do not build/install python support

dtc: do not build/install python support

The pylibfdt in dtc is new since 1.4.5, and therefore built since the
bump from 1.4.4 to 1.4.7 in commit
7b929ddcf0812310d0113cf3675429dbd123aa99. Unfortunately, this Python
support causes a number of build failures, be it for host-dtc or dtc.

Since this Python support is not used/needed by anything in Buildroot
at this point, let's simply disable it by passing NO_PYTHON=1. While
doing this, introduce DTC_MAKE_OPTS and HOST_DTC_MAKE_OPTS to avoid
duplicating the variables several times.

Fixes:

  http://autobuild.buildroot.net/results/1ad41fd4890af0c97ee4ff2994ea5431b8953ef8/ (host-dtc)
  http://autobuild.buildroot.net/results/0aefc3aa03a93b1fdf76f2d8946b6f423405382f/ (dtc)

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