Common code cleanup

[solidpixel]

A collection of source code cleanups for the generated code:

• Consistent Doxygen style (must use @ everywhere).
• Consistent include path style (must include prefix directory for common code).
• Consistent license header (fix one missing use of Arm proprietary preamble I missed).
• Moves CMake compiler helper to common .cmake file.
• Allows layer code to write over existing layer code to facilitate updates.