Allow selinux to be set by bootconfig

The androidboot.selinux property is loaded in a special way, because it
happens in the "selinux_setup" stage, and not the true second stage.
Allow it to be passed through bootconfig instead of only via the kernel
cmdline.

Bug: 173815685
Test: launch_cvd -extra_kernel_cmdline androidboot.selinux=permissive
Test: launch_cvd -guest_enforce_security=false [bootconfig method]
[..]
init: Permissive SELinux boot, forcing sys.init.perf_lsm_hooks to 1.
[..]
Change-Id: I92003c7a2dac5d6e7d0e0f4ee2757f86cc0087c7