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>