configs: ks2: move CMD_NAND to defconfigs

NAND is not yet enabled on all Keystone2 platforms. So enabled
CMD_NAND in the respective defconfigs only if available.

Reported-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

configs/k2e_evm_defconfig

@@ -16,3 +16,4 @@ CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_DM_ETH=y
 CONFIG_SYS_NS16550=y
 CONFIG_TI_AEMIF=y
+CONFIG_CMD_NAND=y

configs/k2hk_evm_defconfig

@@ -16,3 +16,4 @@ CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_DM_ETH=y
 CONFIG_SYS_NS16550=y
 CONFIG_TI_AEMIF=y
+CONFIG_CMD_NAND=y

configs/k2l_evm_defconfig

@@ -16,3 +16,4 @@ CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_DM_ETH=y
 CONFIG_SYS_NS16550=y
 CONFIG_TI_AEMIF=y
+CONFIG_CMD_NAND=y

include/configs/ti_armv7_keystone2.h

@@ -208,7 +208,6 @@
 #define CONFIG_CMD_DHCP
 #define CONFIG_CMD_PING
 #define CONFIG_CMD_SAVES
-#define CONFIG_CMD_NAND
 #define CONFIG_CMD_UBI
 #define CONFIG_CMD_UBIFS
 #define CONFIG_CMD_SF