debug: tracing: Add Segger RTT linker section options

Allows optionally placing Segger RTT data either in a specific
linker section that is located at RAM start, or in a specific linker
section defined by a memory region in DTS, as third and fourth
alternative to the DTCM section or the default data section.
This is useful to share the fixed address for different programs,
typically bootloader and application, and have seamless logging.

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