just plain cp /proc/cmdline

this removes the bash-ism
```
`< …`
```

Author: gdamjan

secure-boot

@@ -52,7 +52,7 @@ $(BUILDDIR)/cmdline.txt:
 	@mkdir -p $(BUILDDIR)
 	[ -f ${CMDLINE} ] && \
 	    tr '\n' ' ' < ${CMDLINE} > $@ || \
-	    echo -n `</proc/cmdline` > $@
+	    cp /proc/cmdline $@
 
 $(BUILDDIR)/initramfs.img: $(UCODE) $(INITRAMFS)
 	@mkdir -p $(BUILDDIR)