configs/olimex_a20_olinuxino_lime{, 2}: use Python 3.x

configs/olimex_a20_olinuxino_lime{, 2}: use Python 3.x

The olimex_a20_olinuxino_lime{,2}_defconfig uses U-Boot 2020.04 since
commit 6b805c3ab70b5ee63c7dcd3a4aa48a999a8a43c3. This new U-Boot
version needs Python 3.x for pylibfdt.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/535054468
  https://gitlab.com/buildroot.org/buildroot/-/jobs/535054466

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr: update with the new NEEDS_PYTHON semantics]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>