[yocto] possible to append or patch existing machine .conf file?

[Bernd]

Suppose I have the following bugfix patch for a 3rd party machine conf
file I am using:

diff --git a/conf/machine/colibri-vf.conf b/conf/machine/colibri-vf.conf
index 3ddef79..ba47488 100644
--- a/conf/machine/colibri-vf.conf
+++ b/conf/machine/colibri-vf.conf
@@ -35,6 +35,6 @@ MKUBIFS_ARGS = " -c 8112 -e 124KiB -m 2KiB -F"
 UBINIZE_ARGS = " -p 128KiB -m 2048 -s 2048"
 UBI_VOLNAME = "rootfs"

-SERIAL_CONSOLE ?= "115200 ttyLP0"
+SERIAL_CONSOLE = "115200 ttyLP0"

 MACHINE_FEATURES += "usbgadget usbhost vfat alsa touchscreen"

Is there a proper way to somehow temporarily add something to my layer
to apply this patch until it makes its way upstream and into the
branch I am using? Or should I make my own machine file, include the
original one and then change the variable?

So far I have not found any elegant way to otherwise force this
variable to its correct value in my image recipe, the only way I have
found to work around this bug is to .bbappend the inittab recipe where
this variable is actually used and change the value of another
variable (SERIAL_CONSOLES, note the S at the end) which is derived
from SERIAL_CONSOLE right there in this bbappend file. This seems to
help.

But while doing this I have also noticed a strange anomaly in the
output of bitbake -e:

* When I change the variable in my image recipe then bitbake -e will
show another "set" access and both variables SERIAL_CONSOLE and
SERIAL_CONSOLES will have the correct value as intended by me but the
produced image will have the **wrong** entry in its inittab.

* When I bbappend the inittab recipe to set SERIAL_CONSOLES right
there where it is used then my final image will have a correct inittab
and the serial console will work bit there is **no** mention of that
variable change in the output of bitbake -e

* Only when I change the machine conf file iitself to set the variable
then bitbake -e and the produced image both show the correct entry.
Why does it behave that way?