Bootctrl: Replace usage of ro.build.product with ro.board.platform

As replacing TARGET_PRODUCT with TARGET_BOARD_PLATFORM,
also need replace ro.build.product with ro.board.platform.

Change-Id: I191d8c541e7e59b398dfa945f1cc300f59730224
CRs-Fixed: 2975116
diff --git a/1.1/libboot_control_qti/libboot_control_qti.cpp b/1.1/libboot_control_qti/libboot_control_qti.cpp
index 99b15e8..86887b9 100644
--- a/1.1/libboot_control_qti/libboot_control_qti.cpp
+++ b/1.1/libboot_control_qti/libboot_control_qti.cpp
@@ -55,7 +55,7 @@
 #define BOOT_IMG_PTN_NAME "boot"
 #define LUN_NAME_END_LOC 14
 #define BOOT_SLOT_PROP "ro.boot.slot_suffix"
-#define BOARD_PLATFORM_PROP  "ro.build.product"
+#define BOARD_PLATFORM_PROP  "ro.board.platform"
 #define GVMQ_PLATFORM        "msmnile_gvmq"
 
 #define SLOT_ACTIVE 1