scripts: cube update: Improve log readability and add debug option

Currently, log console was almost unreadable,
and we miss the useful error/warning messages.
Improve readability by:
* only useful messages are displayed on console by default
  output of all git/shell command are masked
* Add a "debug" option to have the full log in case debug
  is necessary (debug is disabled by default)
* Improve log messages

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>