[yocto] Kernel config incremental modification not working.

[Bent Bisballe Nyeng]

Hi list

I am working on a project based on the iMX6UL-EVK using the meta-fsl-arm
layer for the kernel.
In a local layer I have a bbappend recipe containing a patch for an
extra kernel feature (a framebuffer device) in that kernel as well as a
.cfg enabling said feature.
The bbappend recipe is located in
recipes-kernel/linux/linux-fslc-imx_%.bbappend and looks like this:
FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"

SRC_URI += " \
        file://0001-mxc-4.1.patch \
        file://ST7789S.cfg \
"

KERNEL_DEVICETREE = "imx6ul-14x14-evk.dtb"

The .cfg is located in recipes-kernel/linux/linux-fslc-imx/ST7789S.cfg
and looks like:
CONFIG_FB_MXS_ST7789S_QVGA=y

The 0001-mxc-4.1.patch patch is correctly applied but the .cfg is either
ignored or overwritten by some later buyild step since the resulting
.config after kernel compilation contains:
# CONFIG_FB_MXS_ST7789S_QVGA is not set

I have tried finding the script in the build/.../temp folder that takes
care of the .cfg file patching but have not been able to find anything
useful.

Any hints as to where I should start looking for a solution?

Kind regards
Bent Bisballe Nyeng