configs/cubieboard2: U-Boot needs pylibfdt
When building U-Boot for this platform:
unable to execute 'swig': No such file or directory
error: command 'swig' failed with exit status 1
scripts/dtc/pylibfdt/Makefile:27: recipe for target 'scripts/dtc/pylibfdt/_libfdt.so' failed
Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/123771048
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>