x86/kexec: Cleanup KEXEC_VERIFY_SIG Kconfig help text

Fix typos and also make it simpler without losing the gist of what it says.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vivek Goyal <vgoyal@redhat.com>
Link: http://lkml.kernel.org/r/1426251877-11415-1-git-send-email-bp@alien8.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>