dtc: Use pkg-config to locate libyaml

Using Makefile's wildcard with absolute path to detect
the presence of libyaml results in false-positive
detection when cross-compiling e.g. in yocto environment.
The latter results in build error:
| scripts/dtc/yamltree.o: In function `yaml_propval_int':
| yamltree.c: undefined reference to `yaml_sequence_start_event_initialize'
| yamltree.c: undefined reference to `yaml_emitter_emit'
| yamltree.c: undefined reference to `yaml_scalar_event_initialize'
...
Use pkg-config to locate libyaml to address this scenario.

Signed-off-by: Pavel Modilaynen <pavel.modilaynen@axis.com>
[robh: silence stderr]
Signed-off-by: Rob Herring <robh@kernel.org>