kernel: add missing config symbol for linux 3.18

Signed-off-by: Felix Fietkau <[email protected]>

target/linux/generic/config-3.18

@@ -2275,6 +2275,7 @@ CONFIG_MTD_SPLIT=y
 CONFIG_MTD_SPLIT_FIRMWARE_NAME="firmware"
 # CONFIG_MTD_SPLIT_FIT_FW is not set
 # CONFIG_MTD_SPLIT_LZMA_FW is not set
+# CONFIG_MTD_SPLIT_MINOR_FW is not set
 # CONFIG_MTD_SPLIT_SEAMA_FW is not set
 CONFIG_MTD_SPLIT_SQUASHFS_ROOT=y
 CONFIG_MTD_SPLIT_SUPPORT=y