Set environment variable for PDBG_DTB

Override the default PDBG library device tree by setting a device tree
using the PDBG_DTB environment variable.

Signed-off-by: Ben Tyner <ben.tyner@ibm.com>
Change-Id: I012f48cbc3261be95ca47ffd434aa7ebf7d1ed61
diff --git a/attn/attn_handler.service b/attn/attn_handler.service
index 1913fdf..44f4f6a 100644
--- a/attn/attn_handler.service
+++ b/attn/attn_handler.service
@@ -9,6 +9,7 @@
 Conflicts=obmc-host-stop@0.target
 
 [Service]
+@ENABLE_PHAL_TRUE@Environment="PDBG_DTB=/var/lib/phosphor-software-manager/pnor/rw/DEVTREE"
 ExecStart=/usr/bin/openpower-hw-diags --daemon
 Restart=on-failure