a3m071: Add needed dependency on LIB_RAND

Boards using link-local need lib rand.

When link-local is moved to Kconfig, this dependency will be described
there.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

configs/a3m071_defconfig

 CONFIG_PPC=y
+CONFIG_CMD_NET=y
 CONFIG_MPC5xxx=y
 CONFIG_TARGET_A3M071=y
 CONFIG_SPL=y
-CONFIG_CMD_NET=y
+CONFIG_LIB_RAND=y

configs/a4m2k_defconfig

 CONFIG_PPC=y
+CONFIG_CMD_NET=y
 CONFIG_MPC5xxx=y
 CONFIG_TARGET_A3M071=y
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="A4M2K"
-CONFIG_CMD_NET=y
+CONFIG_LIB_RAND=y