Allow custom bootloader msg offset in block misc

Globally define BOARD_RECOVERY_BLDRMSG_OFFSET with a decimal integer
to offset the read/write location in misc where the bootloader message
should appear. Example:

  BOARD_GLOBAL_CFLAGS := -DBOARD_RECOVERY_BLDRMSG_OFFSET=2048

Edify commands get_stage and set_stage need to be aware of the
custom bootloader msg offset because they write the stage directly
to the BCB.

Change-Id: Ifdb5ffe3e893a651be59ae63e3a0ebadd828c9f2
diff --git a/bootloader_message/bootloader_message.cpp b/bootloader_message/bootloader_message.cpp
index 4d1ce5b..449f40e 100644
--- a/bootloader_message/bootloader_message.cpp
+++ b/bootloader_message/bootloader_message.cpp
@@ -159,7 +159,10 @@
     *err = "no misc device set";
     return false;
   }
-  int fd = (open(misc_blk_device.c_str(), O_WRONLY | O_SYNC));
+  int open_flags = O_WRONLY | O_SYNC;
+  if (offset > 0)
+    open_flags = O_RDWR | O_APPEND | O_SYNC;
+  int fd = (open(misc_blk_device.c_str(), open_flags));
   if (fd == -1) {
     *err = "failed to open " + misc_blk_device + ": ";
     *err += strerror(errno);
diff --git a/bootloader_message/include/bootloader_message/bootloader_message.h b/bootloader_message/include/bootloader_message/bootloader_message.h
index 52d7331..e0fc2cd 100644
--- a/bootloader_message/include/bootloader_message/bootloader_message.h
+++ b/bootloader_message/include/bootloader_message/bootloader_message.h
@@ -25,8 +25,13 @@
 // 16K - 64K    Used by uncrypt and recovery to store wipe_package for A/B devices
 // Note that these offsets are admitted by bootloader,recovery and uncrypt, so they
 // are not configurable without changing all of them.
+#ifdef BOARD_RECOVERY_BLDRMSG_OFFSET
+static const size_t BOOTLOADER_MESSAGE_OFFSET_IN_MISC = BOARD_RECOVERY_BLDRMSG_OFFSET;
+static const size_t WIPE_PACKAGE_OFFSET_IN_MISC = 16 * 1024 + BOOTLOADER_MESSAGE_OFFSET_IN_MISC;
+#else
 static const size_t BOOTLOADER_MESSAGE_OFFSET_IN_MISC = 0;
 static const size_t WIPE_PACKAGE_OFFSET_IN_MISC = 16 * 1024;
+#endif
 
 /* Bootloader Message
  *