package/dtc: add optional libyaml dependency

Since bump to version 1.5.0 with commit
b2f4dc7b, dtc binary optionally depends
on libyaml which is enabled if it is found by pkgconfig so:
 - add host-pkgconf and libyaml dependencies only if
   BR2_PACKAGE_DTC_PROGRAMS and BR2_PACKAGE_LIBYAML are set
 - disable yaml for host build (and so remove host-pkgconf from
   HOST_DTC_DEPENDENCIES)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>