scripts/dtc: Fix pkg-config behavior under sysroot

scripts/dtc: Fix pkg-config behavior under sysroot

When building with a toolchain that uses a modified sysroot (such as a
Yocto-generated SDK) that does not include libyaml, on a host that does
have libyaml, building dtc will fail with errors like:

  HOSTLD  scripts/dtc/dtc
/usr/lib/gcc/x86_64-pc-linux-gnu/14/../../../../x86_64-pc-linux-gnu/bin/ld:
scripts/dtc/yamltree.o: in function `yaml_propval_int':
yamltree.c:(.text+0x167): undefined reference to
`yaml_sequence_start_event_initialize'
/usr/lib/gcc/x86_64-pc-linux-gnu/14/../../../../x86_64-pc-linux-gnu/bin/ld:
yamltree.c:(.text+0x172): undefined reference to `yaml_emitter_emit'
/usr/lib/gcc/x86_64-pc-linux-gnu/14/../../../../x86_64-pc-linux-gnu/bin/ld:
yamltree.c:(.text+0x1e8): undefined reference to
`yaml_scalar_event_initialize'
/usr/lib/gcc/x86_64-pc-linux-gnu/14/../../../../x86_64-pc-linux-gnu/bin/ld:
yamltree.c:(.text+0x1f5): undefined reference to `yaml_emitter_emit'

(... rest of errors truncated ...)

This happens because the test looks for the file in the default path but
uses pkg-config, which is affected by changing sysroot, to determine the
correct linker arguments. This does not happen when building entirely
within yocto, as pseudo will intercept and rewrite the file path when
trying to test for /usr/include/yaml.h to match the sysroot and thus
generate consistent behavior.

This commit adds the PKG_CONFIG_SYSROOT_DIR prefix to the file path
in order to test against the same conditions that are used to resolve
the build flags for libyaml.

In linux commit ef8795f3f1c ("dt-bindings: kbuild: Use DTB files for
validation"), including yaml is disabled again anyway because of other
problems that it causes, so this problem can also be addressed by
partially backporting that commit instead and simply disabling the yaml
support.

Fixes: 0535e46d55d7 ("scripts/dtc: Update to upstream version v1.7.2-35-g52f07dcca47c")
Signed-off-by: Greg Malysa <malysagreg@gmail.com>