printf() and print level can be used for debug

The print levels are:

    PR_ERR (=1), PR_WARN (=2), PR_INFO (=3), PR_DEBUG (=4)

Calling printf(PR_INFO "message\n") will set the output message
as print level 3 and get printed if CONFIG_PRINT_LEVEL >= 3.

For convenience, pr_err(), pr_warn(), pr_info() and pr_debug()
can be used to print the according level messages, combineded the
tag `pr_fmt(fmt)' if pr_fmt(fmt) is defined.
For example:

    #define pr_fmt(fmt) "TAG: " fmt
    ...
    pr_info("message\n");

the output will be "TAG: message".

	new file:   include/common.h
	new file:   include/compiler.h
	new file:   include/printf.h
	new file:   include/string.h
	new file:   include/types.h
	modified:   kernel/main.c
	modified:   lib/Makefile
	new file:   lib/printf.c
	new file:   lib/string.c
	modified:   tools/make/conf.sh