package/dtc: fix build without libyaml

- Pass NO_YAML=1 to make when libyaml is disabled
- Move BR2_PACKAGE_LIBYAML block outside
  BR2_PACKAGE_DTC_PROGRAMS block to avoid duplicating NO_YAML=1 even if
  libyaml is only used / needed with dtc program

Fixes:
 - http://autobuild.buildroot.org/results/68a1ba8d615951931567bc8a409c1d16624c902b



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>