blackfin: fix undefined reference to srand and rand

Commit 9ba9e85f3f1c (net: Fix NET_RANDOM_ETHADDR dependencies)
accidentally dropped CONFIG_LIB_RAND defines for 14 Blackfin boards.

Prior to that commit, those boards defined CONFIG_LIB_RAND, but not
CONFIG_NET_RANDOM_ETHADDR.  So, commit 9ba9e85f3f1c should not have
touched them, but in fact it ripped CONFIG_LIB_RAND off from all the
header files, which caused undefined reference to srand and rand.
CONFIG_LIB_RAND=y must be revived for such boards.

BTW, this commit indeed makes it better, but even with this fix,
three boards (bf533-stamp, bf538f-ezkit, cm-bf548) still can not
build due to region 'ram' overflowed error.  This was cause by
commit 6eed3786c68c (net: Move the CMD_NET config to defconfigs)
because CMD_NET selects NET, and NET selects REGEX.  Eventually,
some boards were newly enabled with CONFIG_REGEX, increasing the
memory footprint.  A patch is expected to fix the build error.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/configs/br4_defconfig b/configs/br4_defconfig
index e6970a4..7247b9c 100644
--- a/configs/br4_defconfig
+++ b/configs/br4_defconfig
@@ -2,3 +2,4 @@
 CONFIG_TARGET_BR4=y
 CONFIG_CMD_NET=y
 CONFIG_CC_OPTIMIZE_LIBS_FOR_SPEED=y
+CONFIG_LIB_RAND=y