Update Build Environment to Vivado 2018.2 and Pluto v0.31

[dl1com]

Hi,
I tried to build charon with a toolchain based on Vivado 2018.2. It compiles vanilla pluto firmware v0.31 correctly, but there arise several issues when trying to build charon and its custom configured packages (fftw, liquid-dsp, linux-utils).
I propose to update the project to support Vivado 2018.2 and Pluto v0.31, as it as well gets harder and harder to set up a build environment which supports Vivado 2016 and Pluto 0.28.

[williamSusan-cyber]

I met some troubles when making liquid-dsp(using Vivado 2017.4 and pluto firmware v0.34), and the err info is shown as below:

////
src/equalization/src/eqlms.c:458:30: internal compiler error: in equal, at gimplify.c:11894
float x2_n = crealf(_x * conjf(_x));
^~~~~~~~~
Please submit a full bug report,
with preprocessed source if appropriate.
See https://bugs.linaro.org for instructions.