build/DTC: fix sed usage in DTC command

Current sed usage in the DTC command relies on GNU sed specific -i
option which has a slightly different syntax for BSD sed and always
expects an extension to be provided in order to create a backup file.

Instead drop the cat concatenation done before the sed call and use
sed itself to edit and concatenate the files.

No functional change intended.

Signed-off-by: Roger Pau Monné <royger@FreeBSD.org>

scripts/Makefile.lib

@@ -326,8 +326,7 @@ cmd_dtc = mkdir -p $(dir ${dtc-tmp}) ; \
 		-d $(depfile).dtc.tmp $(dtc-tmp) || \
 		(echo "Check $(shell pwd)/$(pre-tmp) for errors" && false) \
 		; \
-	cat $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile) ; \
-	sed -i "s:$(pre-tmp):$(<):" $(depfile)
+	sed "s:$(pre-tmp):$(<):" $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile)
 
 $(obj)/%.dtb: $(src)/%.dts FORCE
 	$(call if_changed_dep,dtc)