nvidia-kernel-oot: don't sign dtbs that aren't built here

In the case of a custom provider of virtual/dtb, the devicetrees won't
all be built by nvidia-kernel-oot, so it shouldn't die if an item in
the KERNEL_DEVICETREE list isn't found here.

Note that previously, DTBFILE was overwritten for the custom devicetrees,
but due to changes elsewhere, this is no longer possible. The choice was
between patching those other places, or this change, which now allows a
dtb to be built by more intuitively updating the KERNEL_DEVICETREE variable.

Signed-off-by: Kurt Kiefer <kekiefer@gmail.com>

recipes-kernel/nvidia-kernel-oot/nvidia-kernel-oot_36.3.0.bb

@@ -75,7 +75,10 @@ do_compile() {
 
 do_sign_dtbs() {
     for dtbf in ${KERNEL_DEVICETREE}; do
-        tegra_uefi_attach_sign "${B}/nvidia-oot/device-tree/platform/generic-dts/dtbs/${dtbf}"
+        local dtb="${B}/nvidia-oot/device-tree/platform/generic-dts/dtbs/${dtbf}"
+        if [ -e "$dtb" ]; then
+            tegra_uefi_attach_sign "$dtb"
+        fi
     done
 }
 do_sign_dtbs[dirs] = "${B}"