devicetree: auto-disable dts for old kernels

After '6392a63 rpi-base.inc: Use KERNEL_DEVICETREE by default' was
introduced, kernel versions < 3.18 might not be buildable. Since full
device tree support was introduced in 3.18 this change ensures that all
kernel < 3.18 will automatically disable device tree.

Signed-off-by: Petter Mabäcker <petter@technux.se>

classes/linux-raspberrypi-base.bbclass

@@ -0,0 +1,39 @@
+inherit linux-kernel-base
+
+
+def get_dts(d, ver):
+    staging_dir = d.getVar("STAGING_KERNEL_BUILDDIR", True)
+    dts = d.getVar("KERNEL_DEVICETREE", True)
+
+    # d.getVar() might return 'None' as a normal string
+    # leading to 'is None' check isn't enough.
+    # TODO: Investigate if this is a bug in bitbake
+    if ver is None or ver == "None":
+        ''' if 'ver' isn't set try to grab the kernel version
+        from the kernel staging '''
+        ver = get_kernelversion_file(staging_dir)
+
+    if ver is not None:
+        min_ver = ver.split('.', 3)
+    else:
+        return dts
+
+    # Always turn off device tree support for kernel's < 3.18
+    try:
+        if int(min_ver[0]) <= 3:
+            if int(min_ver[1]) < 18:
+                dts = ""
+    except IndexError:
+        min_ver = None
+
+    return dts
+
+
+def split_overlays(d, out):
+    dts = get_dts(d, None)
+    if out:
+        overlays = oe.utils.str_filter_out('\S+\-overlay\.dtb$', dts, d)
+    else:
+        overlays = oe.utils.str_filter('\S+\-overlay\.dtb$', dts, d)
+
+    return overlays