Created attachment 9456 [details]
my config files

There are several places where it could have gone wrong.

* When you make changes to pc_x86_bios_defconfig this won't be taken into account at all unless you run `make pc_x86_bios_defconfig` again.

* When you make changes manually in pc_x86_bios_defconfig, it's possible you made a mistake (e.g. missed a dependency). As a result, your changes may have been removed from the full .config. Check in the .config if those symbols are there. In general, it's best to run `make menuconfig; make savedefconfig` and review the changes, rather than editing the defconfig manually.

* After you make changes to the configuration, the next build will not take into account changes, it will only build any *new* packages you added. In this case, since you changed the linux configuration, you can run `make linux-reconfigure` to refresh the configuration. In general you may need to do a clean build (`make clean; make`).

May be I have some mistakes? 
I do
1) make menuconfig -C buildroot
And select Load "/mnt/ramdisk/my_external_tree/configs/pc_x86_bios_defconfig"
And select Save "/mnt/ramdisk/my_external_tree/configs/pc_x86_bios_defconfig"

2)
make clean -C buildroot
make BR2_EXTERNAL=${MyBaseDir}/my_external_tree -C buildroot pc_x86_bios_defconfig

I get "# configuration written to /mnt/ramdisk/buildroot/.config", so let's look into /mnt/ramdisk/buildroot/.config
BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="$(BR2_EXTERNAL)/board/my_company/my_board/pc_x86_bios_kernel4.19.269_defconfig"
BR2_TARGET_GENERIC_ISSUE="Welcome to Buildroot123"

yes, "123" is my mark! Looks ok. But may be I have a mistake in /mnt/ramdisk/my_external_tree/board/my_company/my_board/pc_x86_bios_kernel4.19.269_defconfig ? I do
make linux-menuconfig -C buildroot 
And select Load "/mnt/ramdisk/my_external_tree/board/my_company/my_board/pc_x86_bios_kernel4.19.269_defconfig"
And select Save "/mnt/ramdisk/my_external_tree/board/my_company/my_board/pc_x86_bios_kernel4.19.269_defconfig"

Now let's build:
3) 
# get absoletly new buildroot from git
rm -Rf buildroot/
git clone https://github.com/buildroot/buildroot

make BR2_EXTERNAL=${MyBaseDir}/my_external_tree -C buildroot pc_x86_bios_defconfig
make -C buildroot

And I see default values, "Welcome to Buildroot", not "Welcome to Buildroot123"... Why?

BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="$(BR2_EXTERNAL)/board/my_company/my_board/pc_x86_bios_kernel4.19.269_defconfig"
                   This is not correct ^^^

The external tree is referenced by $(BR2_EXTERNAL_XYZZY_PATH), where XYZZY is the name you specified in external.desc. You need to edit this file before setting up your project with 

  $ make BR2_EXTERNAL= ... pc_x86_bios_defconfig,

because this variable is transferred to your .config; check the first section of the file.

Created attachment 9461 [details]
good buildroot and kernel configs for x86

This is working config with kernel 4.19.269 (based on pc_x86_64_bios_defconfig)

Not good works, see details https://bugs.busybox.net/show_bug.cgi?id=15271

There is apparently no Buildroot bug here, but instead:

- Andrei you are manually editing the defconfig file. Don't do that, unless you are extremely familiar with Buildroot options. Please use "make menuconfig" to change the configuration.

- As pointed out by Andreas, using $(BR2_EXTERNAL) will not work, you need to use $(BR2_EXTERNAL_xyz_PATH) where "xyz" is the name of your external as specified in the external.desc file.

Since there's no Buildroot bug here, I'm closing this entry.