build: switch to Python 3

build: switch to Python 3

This patch switches Python used in buildroot to Python 3 as Python 2.x
series is going to be EOL relatively soon[1].

 "Being the last of the Python 2.x series, 2.7 will have an extended
  period of maintenance. Specifically, 2.7 will receive bugfix support
  until January 1, 2020. After the last release, 2.7 will receive no
  support."

It seems like Python 3 is going to be needed in order to support Meson
build system which has gained some traction recently.

Build tested on following targets:

 ath79/generic
 imx6/generic
 ipq40xx/generic
 lantiq/xway
 layerscape/armv7 (broken)
 layerscape/armv8_32b (broken)
 layerscape/armv8_64b (broken)
 mvebu/cortexa72
 ramips/mt7621
 ramips/mt7620
 sunxi/cortexa7
 x86/64

Layerscape targets are unfortunately left broken as their ls-rcw
firmware package uses Python 2 and converting it to Python 3 would take
more then reasoanble amount of time (I've tried to fix it already) and
would be better fixed by someone who can even run test it.

Run tested on ath79 (Archer C7 v5), ipq40xx (nbg6617) and x86_64 (QEMU
and apu2).

1. https://www.python.org/dev/peps/pep-0373/

Ref: PR#1937
Cc: Yangbo Lu <yangbo.lu@nxp.com>
Cc: Mathew McBride <matt@traverse.com.au>
Acked-by: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>