Segger RTT: support CONFIG_SEGGER_RTT_INIT_MODE and optimizations#17

cfriedt merged 2 commits intozephyrproject-rtos:masterzephyrproject-rtos/segger:masterfrom

giansta:fix/rtt-cb-common-addrgiansta/segger:fix/rtt-cb-common-addrCopy head branch name to clipboard