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>

scripts/dtc/Makefile

@@ -11,7 +11,7 @@ dtc-objs	+= dtc-lexer.lex.o dtc-parser.tab.o
 # Source files need to get at the userspace version of libfdt_env.h to compile
 HOST_EXTRACFLAGS := -I $(srctree)/$(src)/libfdt
 
-ifeq ($(wildcard /usr/include/yaml.h),)
+ifeq ($(shell pkg-config --exists yaml-0.1 2>/dev/null && echo yes),)
 ifneq ($(CHECK_DTBS),)
 $(error dtc needs libyaml for DT schema validation support. \
 	Install the necessary libyaml development package.)
@@ -19,7 +19,7 @@ endif
 HOST_EXTRACFLAGS += -DNO_YAML
 else
 dtc-objs	+= yamltree.o
-HOSTLDLIBS_dtc	:= -lyaml
+HOSTLDLIBS_dtc	:= $(shell pkg-config yaml-0.1 --libs)
 endif
 
 # Generated files need one more search path to include headers in source tree