kgdboc: Disable all the early code when kgdboc is a module

kgdboc: Disable all the early code when kgdboc is a module

When kgdboc is compiled as a module all of the "ekgdboc" and
"kgdb_earlycon" code isn't useful and, in fact, breaks compilation.
This is because early_param() isn't defined for modules and that's how
this code gets configured.

It turns out that this was broken by commit eae3e19ca930 ("kgdboc:
Remove useless #ifdef CONFIG_KGDB_SERIAL_CONSOLE in kgdboc") and then
made worse by commit 220995622da5 ("kgdboc: Add kgdboc_earlycon to
support early kgdb using boot consoles").  I guess the #ifdef wasn't
so useless, even if it wasn't obvious why it was useful.  When kgdboc
was compiled as a module only "CONFIG_KGDB_SERIAL_CONSOLE_MODULE" was
defined, not "CONFIG_KGDB_SERIAL_CONSOLE".  That meant that the old
module.

Let's basically do the same thing that the old code (pre-removal of
the #ifdef) did but use "IS_BUILTIN(CONFIG_KGDB_SERIAL_CONSOLE)" to
make it more obvious what the point of the check is.  We'll fix
kgdboc_earlycon in a similar way.

Fixes: 220995622da5 ("kgdboc: Add kgdboc_earlycon to support early kgdb using boot consoles")
Fixes: eae3e19ca930 ("kgdboc: Remove useless #ifdef CONFIG_KGDB_SERIAL_CONSOLE in kgdboc")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200519084345.1.I91670accc8a5ddabab227eb63bb4ad3e2e9d2b58@changeid
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>