init: Adding multiple kernels support with kexec

It's pretty common to have several kernel version installed on the
deployed OS. When rebooting with grub this isn't a big issue since grub
will ask a boot time the kernel you want.

On the other hand, when booting with kexec, the current code was
considering that a single kernel was available. In case of multiple
kernels present on the /boot the code was wrong and prevented the system
to boot. A kernel panic occured.

To fix this situation, the current code is doing the following:
- If a single kernel exists, the behavior is unchanged, kexec will boot
  it
- If 2 or more kernels exists :
  -- by default the first one in the list will be booted
  -- user can enforce the kernel version to use by defining KEXEC_KERNEL= variable

The following example show how to boot a particular version of the
kernel :
LABEL eDeploy
	KERNEL vmlinuz
	INITRD initrd SERV=10.0.2.2 ONSUCCESS=kexec KEXEC_KERNEL=3.2.0-4
init: Improving kexec log messages

Let's simplify and prefix messages related to kexec stuff.
This makes a log easier to read and more accurate if something gets
wrong.
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
	...