Update.py: Tidy up Fn headers, make logger work globally
Update.py: Tidy up Fn headers, make logger work globally

The function headers have been updated to follow the standard format
that should be being used for tools in mbed. This is a one line summary
followed by a descriptive block with more detail.
Updated the handling of the main function so that the logger becomes
global and thus works across all the functions. This has been tested
with both the fork and branch options, and for levels INFO and DEBUG.

j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os