Revision 590

Make printk() log to a buffer.
Tested on Qemu and Geode LX.
Benefits of this patch:
- printk() now works directly after printk_buffer_init(), even before
the serial port is set up.
- If all you want is a log, you don't have to bother with serial output.
- A payload can read and analyze the log.
- You can build on this and buffer log until serial is available, then
flush the messages buffered so far.

The printk buffer is configurable with a default-on Kconfig variable.

If you want to dump the buffer from the Qemu monitor after CAR has been
disabled, use this command:
memsave 0x90000 65536 memdump.bin

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Peter Stuge <peter@stuge.se>
Acked-by: Stefan Reinauer <stepan@coresystems.de>
Acked-by: Marc Jones <marc.jones@amd.com>