Add documentation to port API

JerryScript-DCO-1.0-Signed-off-by: László Langó [email protected]

Update the nuttx-stm32f4 target

Related issue: jerryscript-project#1202

JerryScript-DCO-1.0-Signed-off-by: Zsolt Borbély [email protected]

Refactor flag additions in CMakeLists

CMakeLists already contains macros to ease adding compilation and
warning flags. This patch:
* Ensures that they are used whereever possible.
* Adds more macros to help with other flags as well.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss [email protected]