Updating the mass of the Crazyflie

[ArisMorgens]

Currently, CF_MASS is a default parameter that remains constant for each platform independently of the added weight (decks, motors, markers etc.).
This is not a major issue for most of our users since CF_MASS is only used by some non-default controllers: Mellinger, Brescianini and Lee. However, it would be nice to be able to overwrite CF_MASS in Kbuild, so that everyone can use the exact weight of their platform.

Here are some key-points that we would like to include:

1. The default mass for Crazyflie 2.1 and 2.1+ should be 29g.
2. When using the thrust upgrade kit, the default mass should be 32.5g. Thanks to this PR, the thrust upgrade kit now has its own defconfig.
3. In menuconfig, there should be a parameter, probably under the "Platform configuration" submenu, where the user can modify the CF_MASS parameter.
4. When selecting a controller that depends on the mass, there should be a "warning" to also update the CF_MASS parameter.