x86: stop relying on hexdump for image build signature

Fixes #811

patches/openwrt/0110-x86-stop-relying-on-hexdump-for-image-build-signature.patch

+From: Matthias Schiffer <mschiffer@universe-factory.net>
+Date: Sun, 3 Jul 2016 19:15:49 +0200
+Subject: x86: stop relying on hexdump for image build signature
+
+Use perl to generate random number instead.
+See https://github.com/freifunk-gluon/gluon/issues/811
+
+Signed-off-by: Felix Fietkau <nbd@nbd.name>
+
+Backport of LEDE d5ee23ee275fcd6550b0051de59706cc928cdac8
+
+diff --git a/target/linux/x86/image/Makefile b/target/linux/x86/image/Makefile
+index e7e02f1..6f64eba 100644
+--- a/target/linux/x86/image/Makefile
++++ b/target/linux/x86/image/Makefile
+@@ -40,7 +40,7 @@ ifneq ($(GRUB_TERMINALS),)
+   GRUB_TERMINAL_CONFIG := terminal_input $(GRUB_TERMINALS); terminal_output $(GRUB_TERMINALS)
+ endif
+ 
+-SIGNATURE:=$(shell dd if=/dev/urandom bs=4 count=1 2>/dev/null | hexdump -v -e '"%08x"')
++SIGNATURE:=$(shell perl -e 'printf("%08x", rand(0xFFFFFFFF))')
+ ROOTPART:=$(call qstrip,$(CONFIG_TARGET_ROOTFS_PARTNAME))
+ ROOTPART:=$(if $(ROOTPART),$(ROOTPART),PARTUUID=$(SIGNATURE)-02)
+