Missing kexec-tools dependency results in broken kexec

[rjoleary]

Describe the bug
If you do not run apt-get install kexec-tools, then webboot defaults to the Go implementation which does not support 32-bit

To Reproduce
Build webboot without having a C kexec command installs (which kexec is empty).

Expected behavior
The README should mention installing kexec-tools. There should be a warning if it is not installed.