[yocto] how to set particular changes to a default kernel config

[lothar at denx.de]

Dear Yocto Team,

For an ARM based board (MACHINE = "myboard"), I use a default kernel  
config from arch/arm/configs and want now to change some particular  
CONFIG_ options.

Trying to follow the documentation, I currently have the following files:
.
+- linux-acme
|   |
|   +- additional.cfg
|
+- linux-acme_3.8.bb



...in linux-acme_3.8.bb I have
(...)
S = "${WORKDIR}/git"
(...)
KERNEL_DEFCONFIG_myboard = "blabla_defconfig"
do_configure_prepend_myboard() {
      install -m 0644 ${S}/arch/${ARCH}/configs/${KERNEL_DEFCONFIG} \
            ${WORKDIR}/defconfig || die "no default config"
}
SRC_URI_myboard = "git://kernel.ubuntu.com/ubuntu/linux.git;protocol=git \
            file://additional.cfg"
(...)


...and in additional.cfg I have
CONFIG_DEVTMPFS_MOUNT=y
CONFIG_MTD_CMDLINE_PARTS=y
CONFIG_MTD_BLOCK=y
CONFIG_MTD_M25P80=y


When I run something like...
$ bitbake -b /yocto/meta-myboard/recipes-kernel/linux/linux-acme_3.8.bb -f
...it seems to find the .cfg file, since it stoped complaining (after  
I fixed some paths) and now compiles/builds smoothely.

Anyway, I can't see the changes in the .config in
$BDIR/tmp/work/myboard-linux-gnueabi/linux-acme/3.8+..../git/.config

I imagine something like mixing both configs and running "make  
oldconfig" in behind. Anyway before compilation, the changes should be  
in the .config, right?

Questions:
1) How can I add single additional options to a default kernel config?
2) What is the best way to check if the options were applied?
3) Do I need another approach, e.g. through a patch, using echo, or  
using a .scc file (I tried, but with the same result)?

Best Regards,
Lothar Rubusch