[runtime.c] Simplified runtime platform #73 #75
Replaced mainName option with ET_MAIN_FUNCTION_NAME
- Configure main name using c code only
- Deprecated mainName generator option
support platform specific main function
- added explicit user main function in etPlatformLifeCycle.h
- propagate user return code
- removed ET_RUNNER switch
removed obsolete platforms simplified implementation added documentation for custom platform renamed naming-convention.txt to readme