oniro-openharmony-linux: allow running the system without hdmi connected

enable HDMI_FORCE_HOTPLUG to allow running the system without
an hdmi display connected.
HDMI_FORCE_HOTPLUG configuration does not work with vc4-kms-v3d dtoverlay,
therefore using vc4-fkms-v3d instead.

Signed-off-by: Francesco Pham <francesco.pham@huawei.com>

conf/distro/oniro-openharmony-linux.conf

@@ -48,3 +48,10 @@ HOSTTOOLS += "ssh-keygen"
 ENABLE_UART:raspberrypi4-64 ?= "1"
 
 DISTRO_FEATURES += "wifi"
+
+# Allow running the system without hdmi display connected
+DISABLE_VC4GRAPHICS:raspberrypi4-64 = "1"
+RPI_EXTRA_CONFIG:raspberrypi4-64 = ' \n \
+    dtoverlay=vc4-fkms-v3d \n \
+    '
+HDMI_FORCE_HOTPLUG:raspberrypi4-64 = "1"