I think you already know, but it seems to me that something has broken when building the kmods:

Test CONFIG_DEBUG_VIRTUAL feature (TEST_DEBUG_VIRTUAL) [N/m/y/?] (NEW)
Error in reading or end of file.

make[6]: *** [scripts/kconfig/Makefile:71: syncconfig] Error 1
make[5]: *** [Makefile:603: syncconfig] Error 2
make[4]: *** [Makefile:712: include/config/auto.conf.cmd] Error 2
make[4]: Leaving directory '/workdir/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/linux-5.10.56'
make[3]: *** [Makefile:20: /workdir/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/linux-5.10.56/.modules] Error 2
make[3]: Leaving directory '/workdir/openwrt/target/linux/mvebu'
make[2]: *** [Makefile:11: compile] Error 2
make[2]: Leaving directory '/workdir/openwrt/target/linux'
time: target/linux/compile#25.38#8.34#30.09
ERROR: target/linux failed to build.
make[1]: *** [target/Makefile:25: target/linux/compile] Error 1
make[1]: Leaving directory '/workdir/openwrt'
make: *** [/workdir/openwrt/include/toplevel.mk:230: target/compile] Error 2
Error: Process completed with exit code 2.