bitbake.conf: add IMAGE_ROOTFS_SIZE (from OE)

meta/conf/bitbake.conf

@@ -591,6 +591,7 @@ IMAGE_FSTYPES ?= "jffs2"
 PCMCIA_MANAGER ?= "pcmcia-cs"
 DEFAULT_TASK_PROVIDER ?= "task-base"
 MACHINE_TASK_PROVIDER ?= "${DEFAULT_TASK_PROVIDER}"
+IMAGE_ROOTFS_SIZE ?= "65536"
 IMAGE_ROOTFS_SIZE_ext2 ?= "65536"
 IMAGE_ROOTFS_SIZE_ext2.gz ?= "65536"
 IMAGE_ROOTFS_SIZE_ext3 ?= "65536"