meta-google: gbmc-bridge: Fix bashism

The previous change introduced a bashism, but yocto expects that all
shell scripting is POSIX compatible.

Fixes: bf4e27c3408955c214000108001e92c308c1861f
Change-Id: Ifc3cce4d625a876b224853b3b04b1b84b95327b2
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/meta-google/recipes-google/networking/gbmc-bridge.bb b/meta-google/recipes-google/networking/gbmc-bridge.bb
index 50cf0b5..09bf2ae 100644
--- a/meta-google/recipes-google/networking/gbmc-bridge.bb
+++ b/meta-google/recipes-google/networking/gbmc-bridge.bb
@@ -98,7 +98,7 @@
   if [ ! -z "${GBMC_BR_MAC_ADDR}" ]; then
     sfx='${@mac_to_eui64(GBMC_BR_MAC_ADDR)}'
     addr="[Address]\nAddress=${GBMC_ULA_PREFIX}:$sfx/64\nPreferredLifetime=0\n"
-    addr+="[Address]\nAddress=fe80::$sfx/64\nPreferredLifetime=0"
+    addr="$addr[Address]\nAddress=fe80::$sfx/64\nPreferredLifetime=0"
     sed -i "s,@ADDR@,$addr," ${WORKDIR}/-bmc-gbmcbr.network.in
   else
     sed -i '/@ADDR@/d' ${WORKDIR}/-bmc-gbmcbr.network.in