provide easy way to disable kld_list loading

set kld_disbale=y or any value in the boot loader and that will
disable loading of the kld_list.

Differential Revision: https://reviews.freebsd.org/D26939

libexec/rc/rc.d/kld

@@ -41,6 +41,7 @@ stop_cmd=':'
 kld_start()
 {
 	[ -n "$kld_list" ] || return
+	[ -z "$(kenv -q kld_disable 2>/dev/null)" ] || return
 
 	local _kld