pc-bios/s390-ccw: use STRIP variable in Makefile

The docker-run-test-build@debian-s390x-cross target fails with:

strip --strip-unneeded s390-ccw.elf -o s390-ccw.img
strip: Unable to recognise the format of the input file `s390-ccw.elf'

The configure script defines a STRIP makefile variable whose default
value is ${cross_prefix}strip. Let's use it.

We default to using the non-prefixed strip command in case --enable-debug
or --disable-strip was passed to configure during a regular build.
Signed-off-by: NGreg Kurz <groug@kaod.org>
Message-Id: <149623617700.4947.12490877660892961664.stgit@bahia.lan>
Reviewed-by: NLaurent Vivier <lvivier@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>

pc-bios/s390-ccw/Makefile

@@ -21,8 +21,10 @@ build-all: s390-ccw.img
 s390-ccw.elf: $(OBJECTS)
 	$(call quiet-command,$(CC) $(LDFLAGS) -o $@ $(OBJECTS),"BUILD","$(TARGET_DIR)$@")
 
+STRIP ?= strip
+
 s390-ccw.img: s390-ccw.elf
-	$(call quiet-command,strip --strip-unneeded $< -o $@,"STRIP","$(TARGET_DIR)$@")
+	$(call quiet-command,$(STRIP) --strip-unneeded $< -o $@,"STRIP","$(TARGET_DIR)$@")
 
 $(OBJECTS): Makefile