image.mk: export the FILESYSTEM variable to build commands

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45251 3c298f89-4303-0410-b956-a3cf2f4a3e73

include/image.mk

@@ -353,7 +353,7 @@ define Device/ExportVar
   $(1) : $(2):=$$($(2))
 
 endef
-Device/Export = $(foreach var,$(DEVICE_VARS) KERNEL KERNEL_INITRAMFS,$(call Device/ExportVar,$(1),$(var)))
+Device/Export = $(foreach var,$(DEVICE_VARS) KERNEL KERNEL_INITRAMFS FILESYSTEM,$(call Device/ExportVar,$(1),$(var)))
 
 define Device/Check
   _TARGET = $$(if $$(filter $(PROFILE),$$(PROFILES)),install,install-disabled)