configs/kontron_smarc_sal28_defconfig: use Python 3.x for U-Boot build

New U-Boot versions need Python 3.x for pylibfdt.

Fixes:
 - https://gitlab.com/buildroot.org/buildroot/-/jobs/1006924823



Cc: Michael Walle <michael@walle.cc>
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>