[runtime.c] Simplifcation and documentation runtimes #73

[runtime.c] Simplified runtime platform #73 (closed) #75 (closed)

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