tracing: add RAM backend

If Zephyr is running on a coprocessor we might lack I/O
such as uart or usb to output tracing datas but we might
have gigabytes of RAM available.

This patch allows to output trace datas to a ram buffer, which then
may be retrieved using gdb.

e.g:
(gdb) dump binary memory channel0_0 <ram_tracing_start> \
<ram_tracing_end>

Signed-off-by: Julien Massot <julien.massot@iot.bzh>