m68k: Multi-platform EARLY_PRINTK

Make the boot console available to more m68k platforms by leveraging
the head.S debug console.

The boot console is enabled by the "earlyprintk" command line argument
which is how most other architectures do this.

This is a change of behaviour for the Mac but does not negatively impact
the common use-case which is not debugging.

This is also a change of behaviour for other platforms because it means
the serial port stays quiet when CONFIG_EARLY_PRINTK is not enabled. This
is also an improvement for the common use-case.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Stephen N Chivers <schivers@csc.com.au>
[Geert: CONSOLE_DEBUG should depend on CONFIG_FONT_SUPPORT]
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>