configs/roc_pc_rk3399: fix U-Boot dependencies

configs/roc_pc_rk3399: fix U-Boot dependencies

In fact, roc_pc_rk3399 does not need pylibfdt or pyelftools, but only
a host Python interpreter, to run
./arch/arm/mach-rockchip/make_fit_atf.py.

Since upstream U-Boot commit f05d5743567984b4fff6a862fc0f42760ff135da,
this script no longer needs pyelftools. However, since upstream commit
6d06ea34239ab5099783ce588ad4aead96e1fccb (merged in U-Boot 2020.01),
it requires Python 3.x.

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>