EDIT: This post is from 30.09.2013 and should be the "how to" to this post. However, I never released it for a now unknown reason. Because of the interest in this topic, I still will publish it today so that it can stand as some kind of documentation of what I did back then. However, I might not work anymore.

This is the latest Guide.

What are we doing here?
Building OpenWRT Trunk with the offical RPi 3.10 (next) Kernel.

Packages:
You can use some packages of the offical OpenWRT 12.09 Attitude Adjustment repo.
However, most kernel modules and stuff has to be built but yourself, as I
got no repo. Sorry!

Dependencies needed

sudo apt-get install build-essential subversion git-core libncurses5-dev zlib1g-dev gawk flex quilt libssl-dev xsltproc libxml-parser-perl mercurial bzr ecj cvs unzip

Prepare Enviroment (Trunk)

cd ~
mkdir rpi_openwrt
cd rpi_openwrt
git clone git://git.openwrt.org/openwrt.git
cd openwrt
mkdir dl

Install Packages

./scripts/feeds update -a
./scripts/feeds install -a

Check Enviroment

make defconfig
make prereq

Download Kernel and Prepare:

cd /tmp

wget https://github.com/raspberrypi/linux/archive/rpi-3.10.y.tar.gz

tar xfvz rpi-3.10.y.tar.gz
mv linux-rpi-3.10.y linux-3.10.13
tar -cJvf linux-3.10.13.tar.xz linux-3.10.13/
mv linux-3.10.13.tar.xz ~/rpi_openwrt/openwrt/dl/

Prepare Target

cd ~/rpi_openwrt/openwrt/target/linux/brcm2708/

Remove the RPi Kernel specific patches

we don't need them because we use the ORIGINAL kernel

- and not some retailored standard kernel!

mv patches-3.10 patches-3.10.bkp
mkdir patches-3.10

check wheter the makefile does try to catch the right kernel version

vi Makefile

change LINUX_VERSION to LINUX_VERSION:=3.10.13

Prepare Generic Patches

cd ../generic/patches-3.10

Normally it should be ok to just remove some of the patches like this.

if make target/linux/{clean,prepare} V=99 does fail in patching stage

you would need to remove another patch here

but for the time beeing, lets kill these files which are known to

wreck it:

rm 063-arm-fix-fiq-vivt.patch
rm 880-gateworks_system_controller.patch

#Make
cd ~/rpi_openwrt/openwrt/
make menuconfig # Choose BCM2708 Chipset, Raspberry Pi Board, Exit and Save
make target/linux/{clean,prepare} V=99 # does extract kernel and patch it
make kernel_oldconfig # builds some tools
make kernel_menuconfig # configures kernel
make menuconfig # to change RPi Patterns, i.e.:

Packages rep:

Image configuration -> Version configuration options -> Release Repo to:

http://downloads.openwrt.org/attitude_adjustment/12.09/%T/generic/packages

Main Partition Size from 48 to 64:

Target Options -> Root File System Size, change from 48 to 64 MB or more

Exit and Save

make V=99 -j 3

Files are in ~/rpi_openwrt/openwrt/bin/brcm2708
( i.e. openwrt-brcm2708-sdcard-vfat-ext4.img )
New build:
make kernel_menuconfig
make menuconfig
make V=99 -j 3