uml: mark as source-only

Get rid of a special case in the buildbot script

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

target/linux/uml/Makefile

@@ -20,7 +20,7 @@ ARCH:=$(shell uname -m | sed \
 )
 BOARD:=uml
 BOARDNAME:=User Mode Linux
-FEATURES:=squashfs ext4 audio
+FEATURES:=squashfs ext4 audio source-only
 MAINTAINER:=Florian Fainelli <florian@openwrt.org>
 
 KERNEL_PATCHVER:=4.4