init: Improving Kexec kernel reporting

When detecting multiples kernels in kexec mode, let's report the kernel
we found and the one we selected. This is clearly an helper for users to
understand which kernels are availables and which one to specify in
KEXEC_KERNEL variable.

A typical output is :
	...
	Kexec: Multiple kernels found :
	3.8.13.4-desktop-1.mga3
	3.2.0-4-amd64
	Kexec: no KEXEC_KERNEL defined.
	Kexec: Selecting first kernel version : 3.8.13.4-desktop-1.mga3
	...