skiboot: Add BR2_SKIBOOT_DEVICETREE

Introduce a buildroot option to build skiboot's external/devicetree
directory and deliver the binaries as op-build output images.

Signed-off-by: Reza Arbab <arbab@linux.ibm.com>
diff --git a/openpower/configs/opal_defconfig b/openpower/configs/opal_defconfig
index 89f9a8f..a790aef 100644
--- a/openpower/configs/opal_defconfig
+++ b/openpower/configs/opal_defconfig
@@ -49,4 +49,5 @@
 BR2_PACKAGE_PETITBOOT=y
 # BR2_PACKAGE_HOST_P8_PORE_BINUTILS is not set
 # BR2_PACKAGE_CAPP_UCODE is not set
+BR2_SKIBOOT_DEVICETREE=y
 # BR2_PACKAGE_IMA_CATALOG is not set