OpenWrt on Buffalo WHR-600D

I got a Buffalo WHR-600D with DD-WRT (the pro version supplied by Buffalo).
I like the OpenWRT better and want to install it on the router, but firmware upgrading with (linked from the WHR-600D WIKI page) through DD-WRT webinterface does not work - DD-WRT still remain in the flash.
I am familiar with booting the router from TFTP (having unbricked it a couple of times), but I guess that I need an openwrt initramfs image?!? If that is the case, how do I obtain such an image? (I can't find it in the release file structure)

Thanks in advance,

Okay, I compiled the initramfs for the WHR-600D and booted it over tftp - but the router just restarts.
I disassembled the router and soldered on a serial port to see what is going on - It seems that the kernel panics:

[    7.512283] Call Trace:
[    7.517146] [<800d074c>] kfree+0x7c/0x164
[    7.525136] [<82e77020>] rt2x00lib_free_eeprom_file+0x18/0x38 [rt2x00lib]
[    7.538667] [<82e71edc>] rt2x00lib_probe_dev+0x8d8/0x968 [rt2x00lib]
[    7.551327] [<82f7d27c>] rt2x00pci_probe+0x230/0x284 [rt2x00pci]
[    7.563297] [<801ce898>] pci_device_probe+0x98/0x140
[    7.573185] [<801fe888>] really_probe+0x140/0x2b8
[    7.582550] [<801feaa4>] __driver_attach+0xa4/0xdc
[    7.592087] [<801fcab4>] bus_for_each_dev+0x5c/0xbc
[    7.601800] [<801fddf4>] bus_add_driver+0xf8/0x218
[    7.611341] [<801ff320>] driver_register+0xac/0xfc
[    7.620878] [<80004650>] do_one_initcall+0xd0/0x1a0
[    7.630597] [<80078034>] do_init_module+0x74/0x1d0
[    7.640137] [<8007724c>] load_module+0x1f6c/0x2354
[    7.649674] [<80077754>] SyS_init_module+0x120/0x17c
[    7.659565] [<80013fcc>] syscall_common+0x34/0x58
[    7.668925] Code: 30630001  14600002  00000000 <000c000d> 8c430000  7c6303c0  10600003  00000000  10000002 
[    7.688358] 
[    7.691366] ---[ end trace 5aa4d8b89d055647 ]---
[    7.702829] Kernel panic - not syncing: Fatal exception

Any ideas how to proceed?

The complete bootlog is available here:

...and here is the boot log of the Buffalo firmware:

Hmmm... just read How to flash LEDE onto Buffalo WHR-1166D? and just as an experiment I tried booting the WHR-600D with the raminitfs Robin Panda built for Buffalo WHR-1166D and it boots!

However the WAN interface does not seem to work.
Any ideas for getting further with this?

If the LAN interface works, either change the switch config to use a different port as WAN, or download the proper binary in advance to flash after booting.