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
diff --git a/init/selinux.cpp b/init/selinux.cpp
index 0336936..47686b8 100644
--- a/init/selinux.cpp
+++ b/init/selinux.cpp
@@ -92,7 +92,7 @@
 
 enum EnforcingStatus { SELINUX_PERMISSIVE, SELINUX_ENFORCING };
 
-EnforcingStatus StatusFromCmdline() {
+EnforcingStatus StatusFromProperty() {
     EnforcingStatus status = SELINUX_ENFORCING;
 
     ImportKernelCmdline([&](const std::string& key, const std::string& value) {
@@ -101,12 +101,20 @@
         }
     });
 
+    if (status == SELINUX_ENFORCING) {
+        ImportBootconfig([&](const std::string& key, const std::string& value) {
+            if (key == "androidboot.selinux" && value == "permissive") {
+                status = SELINUX_PERMISSIVE;
+            }
+        });
+    }
+
     return status;
 }
 
 bool IsEnforcing() {
     if (ALLOW_PERMISSIVE_SELINUX) {
-        return StatusFromCmdline() == SELINUX_ENFORCING;
+        return StatusFromProperty() == SELINUX_ENFORCING;
     }
     return true;
 }