Allow android_dt_dir to be set by bootconfig

The androidboot.android_dt_dir property is special, because it is loaded
to find out where to get the other DT properties from, and those DT
properties are supposed to override the cmdline/bootconfig ones. So, it
need special casing, and that special case lacked bootconfig support.

Bug: 173815685
Test: launch_cvd -extra_kernel_cmdline androidboot.android_dt_dir=/tmp
[..]
init: Using Android DT directory /tmp
[..]
Change-Id: Ie0958dd0a96394d65f6568653b754ea6f885212e