x86: document fdimage/isoimage completely in make help

Add missing targets and missing options in x86 make help

[ mingo@elte.hu: more whitespace cleanups ]

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>