[OpenPower-Firmware] Requiring Python 2.7

[Joel Stanley]

Hello,

With an upcoming move to Buildroot 2019.03, op-build will require
Python 2.7 to be installed on host machine.

Upcoming change:

 https://github.com/open-power/op-build/pull/2663

Upstream release notes:

 http://lists.busybox.net/pipermail/buildroot/2019-March/244273.html

Please ensure you meet this requirement if you're building firmware.

Cheers,

Joel