Add ability for custom init without making a separate app

[ConnorRigby]

Heres a use case I think Bootloader could solve.

Our FW application has a dependency that relies on code that makes https network requests on start. (Bad practice, but we don't want to rewrite the dep.) The problem is, that app will try to make network requests, fail, then our FW application will never get started since it gets started async while network is still coming up.

config :bootloader,
  init: [:nerves_runtime, :nerves_network, :nerves_ntp, :problem_dep],
  app: :fw

The fix i suggested that worked was to have a separate dep between :nerves_ntp and :problem_dep as a path dependency that blocks until time (and therefor network) is up. but it is very bulky and cumbersome to have a separate app just for three lines of init code, and it also leaves a random process laying around.

Something i think bootloader could solve is laid our something like this:

config :bootloader,
   init: [:nerves_runtime, :nerves_network, :nerves_ntp, Fw.CheckTimeAndNetwork, :problem_dep],
   app: :fw

This would allow for a more robust init system without being to bulky. One thing that will be a problem is the :fw app will not be started so a user would not be able to access any started processes.

I think i remember seeing there might be an API for this already, i just haven't looked and it's not documented.

Thanks @cjfreeze for bringing this up

[entone]

It appears the handler configuration key is meant to handle certain things like this, however, it doesn't appear to be fully implemented.