I had pushed a branch which contains K20D5M target files, it was merged a minute ago to mbed master. It contains exporters for GCC ARM and uVision (ARMCC). I will add a new blog post how to use those exporters, it completely facilitates testing.
The K20D5M is supported, not fully functional. Demo like Hello world works, but for example I2C is not yet complete, neither PWM. Any help with this target is welcomed !
Update your K20D50M to CMSIS-DAP interface, which I shared on my mbed notepad, currently in beta version, link CMSIS-DAP K20D50M [mbed.org/users/Kojto/notebook]
If any questions, leave a comment or contact me on mbed. I will be working on this to complete entire HAL for K20D5M target.