scripts/dtc: consolidate include path options in Makefile

It is tedious to specify extra compiler options for every file.
HOST_EXTRACFLAGS is useful to add options to all files in a
directory.

-I$(src)/libfdt is needed for all the files in this directory
to include libfdt_env.h etc. from scripts/dtc/libfdt/.

On the other hand, -I$(src) is used to include check-in headers
from generated C files.  Thus, I added it only to dtc-lexer.lex.o
and dtc-parser.tab.o .

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>