sgi/install_prerequisites.sh: libfdt: skip python and yaml builds

The libfdt build may fail if you have the following packages installed
 - libyaml-dev
 - python
 - swig

We don't need libfdt for Yaml or Python to build the rd-infra code,
so skip these sections by adding flags to the Makefile.

Signed-off-by: Ryan Harkin <ryan.harkin@arm.com>
Change-Id: I5a775c08c304cfeefce6571e711dd60d60e20f9c