Comment 8 for bug 1489412

sudo cp ~/boot-debian/kernel7.rt.img /boot/uboot/a/vmlinuz
...
reading a/vmlinuz
8546784 bytes read in 5379 ms (1.5 MiB/s)
reading a/initrd.img
5842717 bytes read in 3675 ms (1.5 MiB/s)
Bad Linux ARM zImage magic!

the kernel7.img is a file modified using mkknlimg to boot straight into it (bcm bootloader -> kernel instead of bcm -> uboot -> kernel) and thus a trailer was appended, so you have two options:

1) either you purge the trailer appended by mkknling (https://github.com/raspberrypi/tools/blob/master/mkimage/mkknlimg) - tedious IMO

or

2) you change how snappy boot: modify config.txt and replace "kernel=uboot.bin" with "kernel=kernel7.img", that should be enough (if not, modify the rest of the environment until you get to userspace)

Question: you mentioned you turned on the RT option on that kernel, that means you compiled it, and if that's true, why don't you simply copy the vmlinuz from the compilation tree? That's another option.