[PATCH] powerpc: Add udbg-immortal kernel option

When debugging early kernel crashes that happen after console_init() and
before a proper console driver takes over, we often have to go hack into
udbg.c to prevent it from unregistering so we can "see" what is
happening. This patch adds a kernel command line option "udbg-immortal"
instead to avoid having to modify the kernel.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>