blackfin: fix build error on bct-brettl2 board

Commit 76ec988b062e (net: Remove all calls to net_random_ethaddr())
accidentally deleted CONFIG_TARGET_BCT_BRETTL2=y, and since then
bct-brettl2 would not build.

Since commit a26cd04920dc (arch: Make board selection choices
optional), Kconfig actually allows such a .config file in which no
board is selected, but the build never succeeds.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/bct-brettl2_defconfig b/configs/bct-brettl2_defconfig
index 84eb1fe..3dd3a53 100644
--- a/configs/bct-brettl2_defconfig
+++ b/configs/bct-brettl2_defconfig
@@ -1,4 +1,5 @@
 CONFIG_BLACKFIN=y
+CONFIG_TARGET_BCT_BRETTL2=y
 CONFIG_NET=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED=y