bootloader_message: Add helpers for handling IBootControl MergeStatus.

Move merge_status from bootloader_control_ab, which is in vendor space,
to a new generic AOSP struct in system space. This will allow more
devices to share the same HAL implementation.

This patch also changes libboot_control to compensate for merge_status
moving out of vendor space. The reference HAL library now also provides
separate helper functions for managing the merge status, so devices
using a custom boot control HAL can still take advantage of the new misc
implementation.

Bug: 139156011
Test: manual test
Change-Id: I5cd824e25f9d07aad1476301def5cdc3f506b029

bootloader_message/include/bootloader_message/bootloader_message.h

@@ -185,6 +185,28 @@ static_assert(sizeof(struct bootloader_control) ==
               "struct bootloader_control has wrong size");
 #endif
 
+// Holds Virtual A/B merge status information. Current version is 1. New fields
+// must be added to the end.
+struct misc_virtual_ab_message {
+  uint8_t version;
+  uint8_t merge_status;  // IBootControl 1.1, MergeStatus enum.
+  uint8_t source_slot;   // Slot number when merge_status was written.
+  uint8_t reserved[61];
+} __attribute__((packed));
+
+#define MISC_VIRTUAL_AB_MESSAGE_VERSION 1
+
+#if (__STDC_VERSION__ >= 201112L) || defined(__cplusplus)
+static_assert(sizeof(struct misc_virtual_ab_message) == 64,
+              "struct misc_virtual_ab_message has wrong size");
+#endif
+
+// This struct is not meant to be used directly, rather, it is to make
+// computation of offsets easier. New fields must be added to the end.
+struct misc_system_space_layout {
+  misc_virtual_ab_message virtual_ab_message;
+} __attribute__((packed));
+
 #ifdef __cplusplus
 
 #include <string>
@@ -247,6 +269,10 @@ bool ReadMiscPartitionVendorSpace(void* data, size_t size, size_t offset, std::s
 // offset is in relative to the start of the vendor space.
 bool WriteMiscPartitionVendorSpace(const void* data, size_t size, size_t offset, std::string* err);
 
+// Read or write the Virtual A/B message from system space in /misc.
+bool ReadMiscVirtualAbMessage(misc_virtual_ab_message* message, std::string* err);
+bool WriteMiscVirtualAbMessage(const misc_virtual_ab_message& message, std::string* err);
+
 #else
 
 #include <stdbool.h>