Add a circular memory buffer implementation

This will be used to support console recording. It provides for a circular
buffer which can be written at the head and read from the tail. It supports
avo

Add a circular memory buffer implementation

This will be used to support console recording. It provides for a circular
buffer which can be written at the head and read from the tail. It supports
avoiding data copying by providing raw access to the data.

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

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