main.py: re-work, adding a new WestApp class

The main() function has gotten really long, and it's getting hard to
read. I want to break it up, but there's a fair bit of interdependent
state to manage.

Rather than using global variables, create a WestApp instance to
contain them all and use it instead. This helps with readability
without making zephyrproject-rtos#149 harder to do.

We'll extend this later to handle manifest imports.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>