Fix up K20 layout

[bharrisau]

The Kinetis flash layout is more complicated on the devices than the layout.ld suggests. Here is an example from a different project I was working on (https://github.com/bharrisau/cmsis-freescale/blob/master/MKL25Z/Source/GCC/MKL25Z64.ld).

There is a flash protection section at 0x400 that needs to be treated carefully or else the device's flash can be permanently write protected.

Should also confirm that other platforms do not have this issue.

• Revise K20 layout
• Start a checklist for new platforms that includes critical checks such as flash protection

[errordeveloper]

The example you have pointed at is pretty similar to a full linker script for TI Tiva C also.