scripts/dtc: use pkg-config to include <yaml.h> in non-standard path

Commit 067c650c ("dtc: Use pkg-config to locate libyaml") added
'pkg-config --libs' to link libyaml installed in a non-standard
location.

yamltree.c includes <yaml.h>, but that commit did not add the search
path for <yaml.h>. If /usr/include/yaml.h does not exist, it fails to
build. A user can explicitly pass HOSTCFLAGS to work around it, but
the policy is not consistent.

There are two ways to deal with libraries in a non-default location.

[1] Use HOSTCFLAGS and HOSTLDFLAGS for additional search paths for
    headers and libraries.
    They are documented in Documentation/kbuild/kbuild.rst

    $ make HOSTCFLAGS='-I <prefix>/include' HOSTLDFLAGS='-L <prefix>/lib'

[2] Use pkg-config

    'pkg-config --cflags' for querying the header search path
    'pkg-config --libs'   for querying the lib and its path

If we go with pkg-config, use [2] consistently. Do not mix up
[1] and [2].

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>