Segger RTT: support CONFIG_SEGGER_RTT_INIT_MODE and optimizations

Support Zephyr "RTT Initialization mode" config choice, allowing
Control Block initialization to be retained from another program
(e.g. bootloader).
It includes conditional CB init with full check on Control Block ID or
partial one that checks only the first byte, yet improving its logic.
Minor optimizations with static const added.

Signed-off-by: Giancarlo Stasi <giancarlo.stasi.co@gmail.com>