Busybox sources do not include some mtd headers - e.g. mtd-abi.h mtd-user.h jffs2-user.h This breaks compilation (mipsel target) if either CONFIG_FLASH_ERASEALL OR CONFIG_FLASH_UNLOCK OR CONFIG_FLASH_LOCK is enabled, because the headers cannot be found. The problem was overcome by copying over these headers from the mtd-utils sources to the cross-compiler's system include/mtd directory. Shouldn't they be part of the busybox sources?
those headers come from the kernel. Disable those mtd applets if you don't have those headers and/or don't need them.