Introduce ONIRO_FLAVOUR/ONIRO_FLAVOUR_PRETTY

Each distro configurations are now defining DISTRO and DISTRO_NAME based
on ONIRO_FLAVOUR (which in turn is autogenerated from
ONIRO_FLAVOUR_PRETTY). This reduces duplication but also allows to check
for the flavour at build time via querying ONIRO_FLAVOUR.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>