u-boot-tools: Add dependency on libyaml for dtschema validation

u-boot-tools: Add dependency on libyaml for dtschema validation

yamltree host tool, used to emit YAML from the DT, is now
built as part of the normal u-boot-tools build to support
the schema infrastructure

Fixes link errors like

| /usr/bin/ld: scripts/dtc/yamltree.o: in function `dt_to_yaml':
| yamltree.c:(.text+0x91b): undefined reference to `yaml_emitter_initialize'
| /usr/bin/ld: yamltree.c:(.text+0x926): undefined reference to `yaml_emitter_set_output_file'
| /usr/bin/ld: yamltree.c:(.text+0x933): undefined reference to `yaml_stream_start_event_initialize'
| /usr/bin/ld: yamltree.c:(.text+0x93e): undefined reference to `yaml_emitter_emit'
| /usr/bin/ld: yamltree.c:(.text+0x957): undefined reference to `yaml_document_start_event_initialize'
| /usr/bin/ld: yamltree.c:(.text+0x962): undefined reference to `yaml_emitter_emit'
| /usr/bin/ld: yamltree.c:(.text+0x983): undefined reference to `yaml_sequence_start_event_initialize'
| /usr/bin/ld: yamltree.c:(.text+0x98e): undefined reference to `yaml_emitter_emit'
| /usr/bin/ld: yamltree.c:(.text+0x9ab): undefined reference to `yaml_sequence_end_event_initialize'
| /usr/bin/ld: yamltree.c:(.text+0x9b6): undefined reference to `yaml_emitter_emit'
| /usr/bin/ld: yamltree.c:(.text+0x9c8): undefined reference to `yaml_document_end_event_initialize'
| /usr/bin/ld: yamltree.c:(.text+0x9d3): undefined reference to `yaml_emitter_emit'
| /usr/bin/ld: yamltree.c:(.text+0x9e3): undefined reference to `yaml_stream_end_event_initialize'
| /usr/bin/ld: yamltree.c:(.text+0x9ee): undefined reference to `yaml_emitter_emit'
| /usr/bin/ld: yamltree.c:(.text+0x9fa): undefined reference to `yaml_emitter_delete'

Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>