flavours: use weak (?=) assignment for DISTRO

Strong (=) assignment makes it impossible to override DISTRO
in command line when calling bitbake e.g.:

  $ DISTRO=myCoolOS bitbake myCoolImage

With strong assignment, the local.conf DISTRO setting has
precendence over command line setting. Using weak assignment
allows user to build other DISTRO without editing local.conf.

Signed-off-by: Wojciech Zmuda <wojciech.zmuda@huawei.com>