I am really REALLY stuck - I don't know what kernel image to use with bootm (and where to load it and quite a few other things but getting the right image would be a good start)
I think bootm wants uImage. i.e., the "u-boot image". There is no rule for "make uImage" for x86, but you can run "mkimage" by yourself.
The image format (thus uImage) is just a 64 byte header in front of a binary file, with magic number, lenght, checksum and other information.
Actually, common/cmd_bootm.c::bootm_start() calls boot_get_kernel, which uses img_addr to look for the format. genimg_get_format then is in common/image.c, where image_header_t is used as defined in include/image.h. I admin "uImage" doesn't appear in the functions, though.
Then there is the new FIT format, but I think your want to start with the legacy format.
Hope this helps /alessandro