Merge remote-tracking branch 'bonzini/configure' into staging

# By Peter Maydell (3) and Ákos Kovács (2)
# Via Paolo Bonzini
* bonzini/configure:
  ui/Makefile.objs: delete unnecessary cocoa.o dependency
  default-configs/: CONFIG_GDBSTUB_XML removed
  Makefile.target: CONFIG_NO_* variables removed
  rules.mak: New string testing functions
  rules.mak: New logical functions for handling y/n values