apex: mount up apex files into /sbin for library access

This patch uses the loop device to mount files under
/sbin/ from /system/apex in order for the device to
use libraries or other files store inside these files.

Each apex file is mounted over a loop device into a
directory correpsonding to the apex filename under
/sbin/.

Once apex mounting is completed, TWRP will set a
property twrp.apex.loaded to true so that init
can use them as a LD_LIBARY_PATH source.
Change-Id: I69f14a969123ac9cf1afc85b6cf76836cb092fb2

Change-Id: Ica6a7d8e479bcaec8bec4483b5e8d2b45ee105fb

partition.cpp

@@ -1575,12 +1575,13 @@ bool TWPartition::Mount(bool Display_Error) {
 		TWFunc::Exec_Cmd(Command);
 	}
 
+ #ifndef TW_NO_BIND_SYSTEM
 	if (Mount_Point == "/system_root") {
 		unlink("/system");
 		mkdir("/system", 0755);
 		mount("/system_root/system", "/system", "auto", MS_BIND, NULL);
 	}
-
+ #endif
 	return true;
 }