Use the idle stack to buffer early uses of printk(). Print the buffered characters during initialization when the UART is available and before the idle stack is used normally. This fix relates to a Coverity issue (PW.SET_BUT_NOT_USED).
Update #3678.
This reduces the link-time dependencies and avoids copy-and-paste.
This patch is a part of the BSP source reorganization. Update #3285.
