I can get the RTT mcuboot log by modifing https://github.com/nrfconnect/sdk-mcuboot/blob/main/boot/zephyr/prj.conf#L32

CONFIG_LOG_MODE_MINIMAL=n # former CONFIG_MODE_MINIMAL

And use these config for child_image\mcuboot.conf:

CONFIG_PM_PARTITION_SIZE_MCUBOOT=0x10000

# Disable UART Console and enable the RTT console
CONFIG_UART_CONSOLE=n
CONFIG_RTT_CONSOLE=y
CONFIG_USE_SEGGER_RTT=y

# Config logger
CONFIG_LOG_BACKEND_RTT=y
CONFIG_LOG_BACKEND_UART=n

CONFIG_MULTITHREADING=y

RTT viewer:

The RTT bootload log only shows after programing the image, and RTT viewer needs to connect the DK again to get the app log. This is the normal scenario. If you have two RTT blocks in RAM, RTTViewer will only pick up one. That means it will be a bit random if it picks up MCUboot or the application.