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>

flavours/freertos/local.conf.sample

@@ -20,7 +20,7 @@ MACHINE ??= "qemuarmv5"
 #
 # Default policy config
 #
-DISTRO = "freertos"
+DISTRO ?= "freertos"
 
 #
 # Package Management configuration

flavours/linux/local.conf.sample

@@ -74,7 +74,7 @@ MACHINE ??= "qemux86-64"
 #
 # Default policy config
 #
-DISTRO = "openharmony-linux"
+DISTRO ?= "openharmony-linux"
 
 #
 # Package Management configuration