annotations: support flavour dependencies (flavour inheritance)

annotations: support flavour dependencies (flavour inheritance)

Allow to define flavour inheritance relationship in the annotations
file, such as:

  # FLAVOUR_DEP: {'amd64-lowlatency': 'amd64-generic', 'arm64-lowlatency': 'arm64-generic', 'arm64-lowlatency-64k': 'arm64-lowlatency-64k'}

In this case, for example, -lowlatency flavours inherits the config
values from -generic (both for amd64 and arm64) and -lowlatency-64
inherits the value from -generic-64k (only on arm64).

This allows to strongly reduce the size of annotations and helps to
read and review changes in annotations.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>