u-boot: Fix pylibfdt generation

u-boot: Fix pylibfdt generation

u-boot attempts to build a Python library called pylibfdt. By default,
u-boot would attempt to use the build host's Python interpreter, which
causes numerous problems, not least of which is that it fails if the
host doesn't have the Python development package installed (complaining
about not being able to find Python.h)

Rectify this situation by including the proper build time dependencies
for pylibfdt and passing the proper arguments to make.

[YOCTO #12867]

(From OE-Core rev: 3b0b16300b351878790729d6270cd113bca73eff)

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>