No I didn't rename it. It's as you typed.
i have this option added to com.apple.Boot.plist under Extra folder as well.
<key>Kernel</key>
<string>mach_kernel</string>
I had the same problem after doing some work between my USB (bootable) backup and my normal disk.
My problem was resolved when I went back and corrected the ownership of /Extra/com.apple.Boot.plist and friends (the uid from the external drive was set as the owner because I copied it over when booted in to that install). Hope that might help.
Other thought: as Vermelho is getting at, if the kernel isn't on the root of the filesystem it cannot load. But I'm guessing you haven't moved it.