Support for multiple NuttX targets

[michallenc]

Improve export by adding NuttX current target (arch, chip, board) commit to NuttX mainline adds NUTTX_ARCH, NUTTX_ARCH_CHIP and NUTTX_BOARD variables to NuttX export. We could use this to identify the target in export file and even to support multiple exported targets as a subdirectory of nuttx-targets directory.

Ideally the build for specific target would be selected from GUI, which would also take care of blocks' source code compilation. This way the user would not have to go to CodeGen/nuttx/devices and run make manually. Similar approach could be used for other targets including Linux.