azure-update: Make functions callable from Python scripts

In azure_device_update.py, functions wrapped with the click module can
be called from command line, but they cannot be called from another
Python script like normal functions because click overrides function
parameters.

To allow Azure Device Update functions to be called from another Python
script, put implementation code in regular functions and make click-
wrapped commands call those implementation functions.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>