twrp: use ANDROID_ROOT environment variable

instead of hard-coded /system path

I updated most of the references I found,
but there might be more

For devices that have to mount system at /system_root,
this allows system to be bind mounted to /system
and detected properly by TWRP

Change-Id: I9f142fd8cec392f5b88e95476258dab9c21a9aac
diff --git a/partitions.hpp b/partitions.hpp
index d780fe5..094c718 100644
--- a/partitions.hpp
+++ b/partitions.hpp
@@ -332,6 +332,7 @@
 	void Set_Active_Slot(const string& Slot);                                 // Sets the active slot to A or B
 	string Get_Active_Slot_Suffix();                                          // Returns active slot _a or _b
 	string Get_Active_Slot_Display();                                         // Returns active slot A or B for display purposes
+	string Get_Android_Root_Path();                                           // Returns path of ANDROID_ROOT environment variable
 	struct pollfd uevent_pfd;                                                 // Used for uevent code
 	void Remove_Uevent_Devices(const string& sysfs_path);                     // Removes subpartitions from the Partitions vector for a matched uevent device
 	void Handle_Uevent(const Uevent_Block_Data& uevent_data);                 // Handle uevent data