firmware: Update Kconfig help text for Google firmware

commit d185a346 upstream.

The help text for GOOGLE_FIRMWARE states that it should only be
enabled when building a kernel for Google's own servers.  However,
many of the drivers dependent on it are also useful on Chromebooks or
on any platform using coreboot.

Update the help text to reflect this double duty.

Fixes: d384d6f4

 ("firmware: google memconsole: Add coreboot support")
Reviewed-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Link: https://lore.kernel.org/r/20180618225540.GD14131@decadent.org.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>