When building kernel using in-tree defconfig, set KBUILD_DEFCONFIG, but the
KCONFIG_MODE is not what the documentation says set to alldefconfig.

The documentation says:
An “in-tree” defconfig file will be handled in alldefconfig mode, a
defconfig file placed in ${WORKDIR} through a meta-layer will be handled
in allnoconfig mode"

https://docs.yoctoproject.org/4.0.6/ref-manual/variables.html#term-KCONFIG_MODE

So I made a patch fix it.
Thanks.