dtc: improve reproducibility

Makefile override CFLAGS, so -fmacro/debug-prefix-map
optiions was omitted and binaries contains absolute
patch to sources.

(From OE-Core rev: b58e808a)

Signed-off-by: Oleksiy Obitotskyy <oobitots@cisco.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 21e70c8a7213fae25a38ff7e4a8316a42130d24f)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>