configs/nanopi_neo4: fix U-Boot dependencies

In fact, nanopi_neo4 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>

configs/nanopi_neo4_defconfig

@@ -22,8 +22,7 @@ BR2_TARGET_UBOOT_CUSTOM_VERSION=y
 BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2020.01"
 BR2_TARGET_UBOOT_BOARD_DEFCONFIG="nanopi-neo4-rk3399"
 BR2_TARGET_UBOOT_NEEDS_DTC=y
-BR2_TARGET_UBOOT_NEEDS_PYLIBFDT=y
-BR2_TARGET_UBOOT_NEEDS_PYELFTOOLS=y
+BR2_TARGET_UBOOT_NEEDS_PYTHON3=y
 BR2_TARGET_UBOOT_NEEDS_ATF_BL31=y
 BR2_TARGET_UBOOT_NEEDS_ATF_BL31_ELF=y
 BR2_TARGET_UBOOT_FORMAT_CUSTOM=y