configs/orangepi_plus: U-Boot needs pylibfdt
Since the bump of U-Boot to 2019.01, pylibfdt is needed to be able to
build the U-Boot image, so we need to enable
BR2_TARGET_UBOOT_NEEDS_PYLIBFDT.
Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/199339509
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>