[PATCH] powerpc: Clarify wording for CRASH_DUMP Kconfig option

The wording of the CRASH_DUMP Kconfig option is not very clear. It gives you a
kernel that can be used _as_ the kdump kernel, not a kernel that can boot into
a kdump kernel.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>