compiler: extend flash command to support different output file types

[deadprogram]

This PR addresses #142 by extending the tinygo flash command to support different output file types based on contents of flash key in target file.

For example, the target file microbit.json

"flash": "openocd -f interface/cmsis-dap.cfg -f target/nrf51.cfg -c 'program {hex} reset exit'",

will continue to compile a .hex file becase the token in the flash key is {hex}.

Modifying the target file itsybitsy-m0.json like this:

"flash": "uf2conv.py {bin}"

will compile a .bin file before trying to execute the flashing command.

[aykevl]

It seems to me that this was never really necessary, this works just fine for me:

{
    "flash": "/home/ayke/src/uf2/utils/uf2conv.py {hex}"
}

[deadprogram]

This seemed to be the case for me: microsoft/uf2#3

[aykevl]

Well, it actually works for me. Program seems to work fine.

[deadprogram]

I know I have that problem if I used the bossac bootloader. In any case, it does set up the next part which will be a pure Go solution to replace that Python script.

[aykevl]

Oh that would be really nice. To eliminate dependencies one by one.