oniro.inc: Fix flavour name case in DISTRO_NAME

When DISTRO_NAME was refactored, it reused the lowercase variant of
flavour name changing in this way its style. This fixes it to use
the "pretty" flavour name.

Fixes eclipse/oniro-core/oniro#786



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