Allow overriding MYNEWT_DOWNLOADER from command line

[m-gorecki]

This adds new flag to load and run commands.
The flag (--loader or short -L) allows to override the MYNEWT_DOWNLOADER value from syscfg. Usage example:

--loader stm32_programmer_cli

[andrzej-kaczmarek]

-S can be already used to override syscfg from command line

[m-gorecki]

In case of loading, there is no use overriding other configs than MYNEWT_DOWNLOADER. I assumed it would be better to simply allow user to override only this specific syscfg value. Under the hood it uses the same code to override the value as -S flag, it's just wrapped to not cause confusion

[andrzej-kaczmarek]

my main concern is that this simply overrides syscfg defined in some package - that doesn't sound like a maintainable solution.

would be better if we "simply" add loader packages as a generic solution, i.e. add pkg.type: loader and move all scripts there. bsp would then specify default and/or supported loader(s) which could be overridden from command line.