Replace custom_options with --target

The result is that the "-I" switch is replaced with "-t tud" and "-J"
with "-t novabox".
Simplify generation of bootloader configuration files

Now, every aspect of generation is triggered by command line option
(previously, there were a lot of implicit rules and dependencies).

Removed some configuration options (the rest will follow soon).

Not all combinations of command line switches were tested, so there might
be some errors!