busybox: ensure $(BUSYBOX_BUILD_CONFIG) is writable

If $(BUSYBOX_CONFIG_FILE) is read-only (eg. because Buildroot's
source dir is), the sed fixups to $(BUSYBOX_BUILD_CONFIG) fail.

Fixes: #4363
Signed-off-by: Nathan Lynch <ntl@pobox.com>
[yann.morin.1998@free.fr: use $(INSTALL) instead of cp]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/busybox/busybox.mk

@@ -135,7 +135,7 @@ endef
 endif
 
 define BUSYBOX_COPY_CONFIG
-	cp -f $(BUSYBOX_CONFIG_FILE) $(BUSYBOX_BUILD_CONFIG)
+	$(INSTALL) -D -m 0644 $(BUSYBOX_CONFIG_FILE) $(BUSYBOX_BUILD_CONFIG)
 endef
 
 # Disable shadow passwords support if unsupported by the C library