scripts: Makefile: Remove flags which are not support by EXT_DTC

DTBO relies on external DTC compiler and there are certain flags which
are not supported by the external DTC compiler.
Add a check to remove those flags for now.

Change-Id: I318a1d110bd54a4a888ee8a5456d7c5a39bd9f6b
Signed-off-by: Runmin Wang <runminw@codeaurora.org>