fs: define per-rootfs directory

This will serve in future commits to store pre-rootfs files, like
fakeroot script...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

fs/common.mk

@@ -39,6 +39,8 @@ ROOTFS_USERS_TABLES = $(call qstrip,$(BR2_ROOTFS_USERS_TABLES))
 # all variable references except the arguments must be $$-quoted.
 define inner-rootfs
 
+ROOTFS_$(2)_DIR = $$(FS_DIR)/$(1)
+
 # extra deps
 ROOTFS_$(2)_DEPENDENCIES += host-fakeroot host-makedevs \
 	$$(if $$(PACKAGES_USERS)$$(ROOTFS_USERS_TABLES),host-mkpasswd)