I scanned the source code of u-boot, finding that the ncs0 has been used to select NAND.
enable_gpmc_cs_config(gpmc_config, &gpmc_cfg->cs[0], base, size);
So why Overo route this signal out(J4-pin66:nsc0), even if it has been occupied by NAND?(unuseful)
Does anyone know ncs0's usage? 
Overo hides other un-occupied signals:(ncs2,ncs3,ncs7,useful). Crazy?
There is another bug in u-boot/board/overo.c
ncs5 is used for eth0, chip smc9210,
those lines configured ncs4 after configered ncs5.
 enable_gpmc_cs_config(gpmc_lan_config, &gpmc_cfg->cs[5], 0x2C000000,
 enable_gpmc_cs_config(gpmc_lan_config, &gpmc_cfg->cs[4], 0x2B000000,
Ncs4 is configured but never get initialized. Only ncs5 get initialized. Actually, ncs4 is unused pin, unconnected.
#ifdef CONFIG_SMC911X
 rc = smc911x_initialize(0, CONFIG_SMC911X_BASE);//CONFIG_SMC911X_BASE=0x2C000000
So, nsc4 is configured in a wrong way. If anybody use this pin. Be careful.
At last ,thx for ScottEllis & Danesh Petigara's reply for my former question, "How to compile my own module?"
your adv. is helpful.