Now that this is not in common.h, perhaps it is acceptable to move this
documentation into the header file.

Signed-off-by: Simon Glass <sjg@chromium.org>

From: Sonny Rao <sonnyrao@chromium.org>

These functions are useful in U-Boot because they allow a graceful failure
rather than an unpredictable stack overflow when printf() buffers are
exceeded.

Mostly copied from the Linux kernel. I copied vscnprintf and
scnprintf so we can change printf and vprintf to use the safe
implementation but still return the correct values.

(Simon Glass <sjg@chromium.org> modified this commit a little)

Signed-off-by: Sonny Rao <sonnyrao@chromium.org>

common.h is very large, so before changing the vsprintf functions, move the
prototypes into their own header file.

Signed-off-by: Simon Glass <sjg@chromium.org>