ROS: weak assignment

[vermaete]

Is it possible to make the assignment of the ROS_* variables weak?

diff --git a/conf/distro/petalinux.conf b/conf/distro/petalinux.conf
index c090cf3..aa8cb93 100644
--- a/conf/distro/petalinux.conf
+++ b/conf/distro/petalinux.conf
@@ -12,10 +12,10 @@ DISTRO = "petalinux"
 DISTRO_NAME = "PetaLinux"
 
 # Define the ROS2 Yocto release
-ROS_OE_RELEASE_SERIES = "scarthgap"
+ROS_OE_RELEASE_SERIES ?= "scarthgap"
 
 # Define ROS2 distro release
-ROS_DISTRO = "jazzy"
+ROS_DISTRO ?= "jazzy"
 
 # Follow the Poky format of <Version>+<type>-<date>
 DISTRO_VERSION = "${XILINX_VER_MAIN}+${XILINX_VER_UPDATE}-${XILINX_VER_BUILD}"