Move building blocks of the update into seperate functions.

The approach with awk's rand() wasn't really random between across
devices: When srand() was called without arguments, time() was used as
seed, which of course is the same on all devices when the script is
called via cron at HH:00:00.

This patch instead uses /dev/urandom as source of random (we don't need
cryptographically strong random numbers, so urandom is just fine) but
still uses awk for the comparison as busybox's ash cannot deal with
floats in $(())

Recently the autoupdater contained a case statement to distinguish
between TP-Link WDR3600 and TP-Link WDR4300. This was left over from
previous development versions of autoupdater that relied on the board
name instead of the model. It is thus no longer needed and be removed
safely.

It is now possible to specific multiple branches like this:

config branch stable
  option url ...
  list pubkey ...
  ...

config branch testing
  ...

and select one in autoupdater.settings:

config autoupdater settings
  option branch "stable"