imagebuilder: fix broken image generation with external targets

When using external targets there is a symlink being created for the
target under target/linux which then becomes dangling under Image
Builder. Fix it by dereferencing the possible symlink.

Tested on IB with external target, ipq40xx and mvebu.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/imagebuilder/Makefile

@@ -78,7 +78,7 @@ ifneq ($(CONFIG_SIGNATURE_CHECK),)
 	$(CP) -L $(STAGING_DIR_ROOT)/usr/sbin/opkg-key $(PKG_BUILD_DIR)/scripts/
 endif
 
-	$(CP) $(TOPDIR)/target/linux $(PKG_BUILD_DIR)/target/
+	$(CP) -L $(TOPDIR)/target/linux $(PKG_BUILD_DIR)/target/
 	if [ -d $(TOPDIR)/staging_dir/host/lib/grub ]; then \
 		$(CP) $(TOPDIR)/staging_dir/host/lib/grub/ $(PKG_BUILD_DIR)/staging_dir/host/lib; \
 	fi