linux-user: disable qemu-bridge-helper and socket_scm_helper build

linux-user targets don't need them, and if we ask to build statically
linked binaries, some static libraries they need are not available.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180605160958.5434-1-laurent@vivier.eu>

Makefile

@@ -351,7 +351,7 @@ $(call set-vpath, $(SRC_PATH))
 
 LIBS+=-lz $(LIBS_TOOLS)
 
-HELPERS-$(CONFIG_LINUX) = qemu-bridge-helper$(EXESUF)
+HELPERS-$(call land,$(CONFIG_SOFTMMU),$(CONFIG_LINUX)) = qemu-bridge-helper$(EXESUF)
 
 ifdef BUILD_DOCS
 DOCS=qemu-doc.html qemu-doc.txt qemu.1 qemu-img.1 qemu-nbd.8 qemu-ga.8

tests/Makefile.include

@@ -930,7 +930,7 @@ check-report.html: check-report.xml
 
 # Other tests
 
-QEMU_IOTESTS_HELPERS-$(CONFIG_LINUX) = tests/qemu-iotests/socket_scm_helper$(EXESUF)
+QEMU_IOTESTS_HELPERS-$(call land,$(CONFIG_SOFTMMU),$(CONFIG_LINUX)) = tests/qemu-iotests/socket_scm_helper$(EXESUF)
 
 .PHONY: check-tests/qemu-iotests-quick.sh
 check-tests/qemu-iotests-quick.sh: tests/qemu-iotests-quick.sh qemu-img$(EXESUF) qemu-io$(EXESUF) $(QEMU_IOTESTS_HELPERS-y)