skiboot.mk: use TARGET_MAKE_ENV

The Skiboot makefile was missing the TARGET_MAKE_ENV variable, which
caused it to ignore some of the host-built environment.

The build would also fail if BR2_SKIBOOT_DEVICETREE was set and
dtc was not installed locally.

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
diff --git a/openpower/package/skiboot/skiboot.mk b/openpower/package/skiboot/skiboot.mk
index 0559214..f2b85e2 100644
--- a/openpower/package/skiboot/skiboot.mk
+++ b/openpower/package/skiboot/skiboot.mk
@@ -34,10 +34,10 @@
 
 define SKIBOOT_BUILD_CMDS
 	$(TARGET_CONFIGURE_OPTS) SKIBOOT_VERSION=`cat $(SKIBOOT_VERSION_FILE)` \
-		$(MAKE) $(SKIBOOT_MAKE_OPTS) -C $(@D) all
+		$(TARGET_MAKE_ENV) $(MAKE) $(SKIBOOT_MAKE_OPTS) -C $(@D) all
 
-	$(if $(BR2_SKIBOOT_DEVICETREE), \
-		$(TARGET_CONFIGURE_OPTS) $(MAKE) -C $(@D)/external/devicetree)
+	$(if $(BR2_SKIBOOT_DEVICETREE), $(TARGET_CONFIGURE_OPTS) \
+		$(TARGET_MAKE_ENV) $(MAKE) -C $(@D)/external/devicetree)
 endef
 
 define SKIBOOT_INSTALL_IMAGES_CMDS