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>