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>

configs/kontron_smarc_sal28_defconfig

@@ -26,6 +26,7 @@ BR2_TARGET_UBOOT_CUSTOM_VERSION=y
 BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2021.01"
 BR2_TARGET_UBOOT_BOARD_DEFCONFIG="kontron_sl28"
 BR2_TARGET_UBOOT_NEEDS_DTC=y
+BR2_TARGET_UBOOT_NEEDS_PYTHON3=y
 BR2_TARGET_UBOOT_NEEDS_PYLIBFDT=y
 # BR2_TARGET_UBOOT_FORMAT_BIN is not set
 BR2_TARGET_UBOOT_FORMAT_CUSTOM=y