printk: collect printk stuff into <linux/printk.h> with loglevel support

When we import code from Linux, with regular re-sync planned, we want
to use printk() and pr_*().  U-Boot does not support them in a clean
way.  So, people end up with local macros, or compat headers here and
there, then we occasionally see build errors of definition conflicts.

We have include/linux/compat.h, but putting all sorts of unrelated
things into a single header is just a temporal workaround.  Hence this
patch, to find the best home for all printk variants.  If you want to
use printk() and friends, please include <linux/printk.h>.  This header
is self-contained, and pulls in only a few headers.

When I was testing this clean-up, I noticed the image size exceeded
its platform limit on some boards.  This is because all pr_*() that
were previously defined as no-op in include/linux/mtd/mtd.h (unless
CONFIG_MTD_DEBUG is set), are now enabled.

To make such boards happy, this commit also implements CONFIG_LOGLEVEL.
The concept is similar to the kernel parameter "loglevel".  (Actually,
the Kconfig help message was taken from kernel-paremeter.txt of Linux)
Messages with a loglevel smaller than console loglevel will be printed.

The difference is the loglevel is build-time determined.  To save the
image size, lower priority pr_*() are compiled out.  I set the default
of CONFIG_LOGLEVEL to 6, i.e. pr_notice and higher priority messages
are compiled in.

I adjusted CONFIG_LOGLEVEL to avoid build error for some boards.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
[trini: Add in SPL_LOGLEVEL that is the same as LOGLEVEL]
Signed-off-by: Tom Rini <trini@konsulko.com>

include/common.h

@@ -25,6 +25,7 @@ typedef volatile unsigned char	vu_char;
 #include <linux/bitops.h>
 #include <linux/delay.h>
 #include <linux/types.h>
+#include <linux/printk.h>
 #include <linux/string.h>
 #include <linux/stringify.h>
 #include <asm/ptrace.h>
@@ -55,11 +56,6 @@ typedef volatile unsigned char	vu_char;
 #define _SPL_BUILD	0
 #endif
 
-/* Define this at the top of a file to add a prefix to debug messages */
-#ifndef pr_fmt
-#define pr_fmt(fmt) fmt
-#endif
-
 /*
  * Output a debug text when condition "cond" is met. The "cond" should be
  * computed by a preprocessor in the best case, allowing for the best