Update README and default settings

Update tool versions. Set topic-miami-7015 as the default machine.
Update the README documentation.
my-image: Fix kernel-module-rsi

kernel-module-rsi is integrated in the 3.15 kernel, but uses a slightly
different name. Adjust the my-image recipe to account for this on the
miami boards.
Add README-JTAGBOOT containing hints on how to boot using JTAG only.