Debugging

Hardware

UART

You can use for debugging purposes early output from UART3 - over the micro-usb port and usb-serial adaptor. The signal is compatible with serial uart devices that work with 3,3V TTL levels (signal levels from the phone are about 2.8V so you have to check specifications of the chosen adaptor to find out if it would work). If you obtain some usb to serial device it is useful to have the usb +5V wired to power the handset while using serial.

Details you can see here: UART

JTAG

JTAG

Software

Great solution for full-feature emulation/debugging : QEMU

Great solution for emulating/debugging/reversing windows-utilities : Wine