blackfin: fix build error on bct-brettl2 board

Commit 76ec988b (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 a26cd049 (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>