build: Propagate user-defined flags to host packages

depends/hosts/default.mk

@@ -29,8 +29,13 @@ host_$1=$$($(host_arch)_$(host_os)_$1)
 endef
 
 define add_host_flags_func
+ifeq ($(filter $(origin $1),undefined default),)
+$(host_arch)_$(host_os)_$1 =
+$(host_arch)_$(host_os)_$(release_type)_$1 = $($1)
+else
 $(host_arch)_$(host_os)_$1 += $($(host_os)_$1)
 $(host_arch)_$(host_os)_$(release_type)_$1 += $($(host_os)_$(release_type)_$1)
+endif
 host_$1 = $$($(host_arch)_$(host_os)_$1)
 host_$(release_type)_$1 = $$($(host_arch)_$(host_os)_$(release_type)_$1)
 endef