Use common location and names for target binaries output

[andrzej-kaczmarek]

Currently binaries generated for target (.elf, .bin, .img etc.) are always put in the same folder as application package. It would be more convenient to put them in common location like root of target build folder and use common name like 'mynewt.*'. This way it would be much easier to locate these files especially when using scripts to do so.

The new location should be likely an option in newtrc and disabled by default for now to make sure we don't break existing workflows.

The base file name could be also configurable via command line (syscfg?) to allow override e.g. for bootloader.