mgmt/mcumgr/lib: Deprecate CONFIG_OS_MGMT_ECHO_LENGTH
mgmt/mcumgr/lib: Deprecate CONFIG_OS_MGMT_ECHO_LENGTH

The echo sentence is now directly copied from request transport
buffer to response buffer, without use of intermediate buffer.

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr