kexec-lite: Depends on elfutils instead of libelf to get the libelf library
The elfutils package provides a more recent version of the libelf, so
let's use it. It will allow to remove the libelf package and to avoid
conflicts with two packages providing the same library.
[Peter: adjust toolchain options comment to match] Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com> Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr> Signed-off-by: Peter Korsgaard <peter@korsgaard.com>