blackfin: fix undefined reference to srand and rand

Commit 9ba9e85f (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 9ba9e85f 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 6eed3786 (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>