[source] brcm63xx: fix build with per-device rootfs

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/?p=source.git;a=commitdiff;h=df68d6eb15bfb3abe97c4048e57bd5192d796edb

commit df68d6eb15bfb3abe97c4048e57bd5192d796edb
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Sun Jul 31 13:17:44 2016 +0200

    brcm63xx: fix build with per-device rootfs
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 target/linux/brcm63xx/image/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/linux/brcm63xx/image/Makefile b/target/linux/brcm63xx/image/Makefile
index ca1792a..f5de673 100644
--- a/target/linux/brcm63xx/image/Makefile
+++ b/target/linux/brcm63xx/image/Makefile
@@ -98,7 +98,7 @@ $(shell expr substr "$(1)" 1 16)
 endef
 
 define Image/FileSystemStrip
-$(subst root.,,$(notdir $(1)))
+$(firstword $(subst +,$(space),$(subst root.,,$(notdir $(1)))))
 endef
 
 define Build/cfe-bin