Replace ETL_COMPILER_ICCAVR by ETL_COMPILER_IAR. (#602)
The current definition mechanism for ETL_COMPILER_ICCAVR does not work. Both IAR compilers, for ARM and AVR define `__IAR_SYSTEMS_ICC__`. Thus `ETL_COMPILER_TYPE_DETECTED` will be defined in line before ETL_COMPILER_ICCAVR is defined. This switch will never be entered.
Currently I see no reason for differentiating both compilers (`__ICCARM__` and `__ICCAVR__`). The condition for the IAR compiler platform (`__IAR_SYSTEMS_ICC__`) is sufficient (combined with <C++11 detection).
At the moment ETL_COMPILER_ICCAVR is used as a switch condition for using `#pragma push_macro`. But actually IAR ARM and IAR AVR have no such macro defined. ETL_COMPILER_IAR is defined for both compilers. Thus the switch condition is replaced with ETL_COMPILER_IAR.
