On a minimalistic VM, I got

# kexec-bootloader 
kexec not available. Install kexec-tools.

This is because it uses "which kexec" to determine if the kexec binary is
available.

I would propose to just drop that check, because both come from the same
kexec-tools package.

Alternatives would be
1:
to add  Requires: which

2:
to use the fallback which defined in /etc/profile.d/alias.bash with
system("bash -i -c \"which kexec\" &>/devnull")

3:
use perl libraries for that check.
Unfortunately File::Which is not part of the default distribution.

4:
just call kexec --help to see if it works