[yocto] how to build external firmware for kernel

[lothar at denx.de]

Hello Yocto Comunity,

I have a linux kernel and a external firmware. Building manually,  
first I have to checkout and build the firmware "native" to obtain a  
firmware blob, then I need this firmware blob to build the kernel for  
the target.

So far I tried to do this in yocto with separate recipes: a  
firmware.bb and a mylinux.bb. In firmware.bb, I check out the firmware  
via git and let it run "make" on native (this is necessary, since the  
Makefile builds a tool which then automatically gets called inside the  
Makefile to create the firmware blob).

In the kernel recipe, I declare a DEPEND on firmware.bb. I set the  
.config options to load an extra firmware blob. But now, I have a  
problem: I need actually to have the firmware blob within the kernel's  
file hierarchy. Ideally I would copy/move it from the firmware's  
destination directory into the kernel's source directory.

Questions:
How can I achieve this kind of setup working out with yocto or  
bitbake, respectively? What might be the best approach in my situation?

Thanks in advance,
Lothar