Support installing Magisk on vendor_boot

Close #9238, fix #8835

app/core/src/main/java/com/topjohnwu/magisk/core/Info.kt

@@ -47,6 +47,8 @@ object Info {
         private set
     var slot = ""
         private set
+    var isVendorBoot = false
+        private set
     @JvmField val isZygiskEnabled = System.getenv("ZYGISK_ENABLED") == "1"
     @JvmStatic val isFDE get() = crypto == "block"
     @JvmStatic var ramdisk = false
@@ -113,6 +115,7 @@ object Info {
         crypto = getVar("CRYPTOTYPE")
         slot = getVar("SLOT")
         legacySAR = getBool("LEGACYSAR")
+        isVendorBoot = getBool("VENDORBOOT")
 
         // Default presets
         Config.recovery = getBool("RECOVERYMODE")

app/core/src/main/java/com/topjohnwu/magisk/core/tasks/MagiskInstaller.kt

@@ -81,10 +81,12 @@ abstract class MagiskInstallImpl protected constructor(
     }
 
     private fun findImage(slot: String): Boolean {
-        val bootPath = (
-            "(RECOVERYMODE=${Config.recovery} " +
-            "SLOT=$slot find_boot_image; " +
-            "echo \$BOOTIMAGE)").fsh()
+        val cmd =
+            "RECOVERYMODE=${Config.recovery} " +
+            "VENDORBOOT=${Info.isVendorBoot} " +
+            "SLOT=$slot " +
+            "find_boot_image; echo \$BOOTIMAGE"
+        val bootPath = ("($cmd)").fsh()
         if (bootPath.isEmpty()) {
             console.add("! Unable to detect target image")
             return false