Add Vivado power optimization option flag

[sebinho]

This proposal adds a flag to enable power optimization during implementation stage of a Vivado run. Power optimization can have huge impacts, especially when using a device that has a PS (Processing System).

To use this new option, simply add the following flag in your *.core file:

flow_options :
            part           : xczu3eg-sfvc784-1-e
            power_opt      : 1

If set to 0, it will disable the power optimizations (and also if the options is not specified at all)