machine: use weak default for kernel and KERNEL_DEVICETREE settings

To allow other layers to easily provide a default kernel preference to
use with the Raspberry Pi MACHINEs, switch to use the ??= assignment
operator for setting the default kernel and the device tree value in
this layer.

This change is motivated by enabling Xen support in meta-virtualization
to use the raspberrypi4-64 MACHINE definition from this layer with the
Yocto Linux kernels.

Signed-off-by: Christopher Clark <christopher.w.clark@gmail.com>

2 files changed, 2 insertions, 2 deletions

diff --git a/conf/machine/include/rpi-base.inc b/conf/machine/include/rpi-base.inc
index 18368c6..f537188 100644
--- a/conf/machine/include/rpi-base.inc
+++ b/conf/machine/include/rpi-base.inc
@@ -57,7 +57,7 @@ RPI_KERNEL_DEVICETREE ?= " \
     bcm2710-rpi-cm3.dtb \
     "
 
-KERNEL_DEVICETREE ?= " \
+KERNEL_DEVICETREE ??= " \
     ${RPI_KERNEL_DEVICETREE} \
     ${RPI_KERNEL_DEVICETREE_OVERLAYS} \
     "
diff --git a/conf/machine/include/rpi-default-providers.inc b/conf/machine/include/rpi-default-providers.inc
index 181b936..c02d248 100644
--- a/conf/machine/include/rpi-default-providers.inc
+++ b/conf/machine/include/rpi-default-providers.inc
@@ -1,6 +1,6 @@
 # RaspberryPi BSP default providers
 
-PREFERRED_PROVIDER_virtual/kernel ?= "linux-raspberrypi"
+PREFERRED_PROVIDER_virtual/kernel ??= "linux-raspberrypi"
 PREFERRED_PROVIDER_virtual/xserver ?= "xserver-xorg"
 PREFERRED_PROVIDER_virtual/egl ?= "${@bb.utils.contains("MACHINE_FEATURES", "vc4graphics", "mesa", "userland", d)}"
 PREFERRED_PROVIDER_virtual/libgles2 ?= "${@bb.utils.contains("MACHINE_FEATURES", "vc4graphics", "mesa", "userland", d)}"