Android系统 —— Recovery子系统下自动检测U盘内升级包并进行升级

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1. bootable/recovery/recovery.cpp

(1) 下面宏变量 USB_BLK_PATH 是U盘节点路径，使用时按实际情况修改。
(2) 函数 usb_mount_thread 中的 max_mount_cnt 变量规定了尝试挂载U盘的次数，每次循环使用2秒延迟，按照实际情况修改。
(3) 当检测到U盘内有名为 force_ota.zip 升级包文件，则执行升级过程。升级成功后，重启到主系统；否则，重启Recovery系统。

diff --git a/recovery.cpp b/recovery.cpp
--- a/recovery.cpp
+++ b/recovery.cppstatic constexpr const char* LAST_LOG_FILE = "/cache/recovery/last_log";static constexpr const char* LOCALE_FILE = "/cache/recovery/last_locale";#define UFS_DEV_SDCARD_BLK_PATH "/dev/block/mmcblk0p1"
+#define USB_BLK_PATH "dev/block/sda1"+std::string force_ota_path = "";static constexpr const char* CACHE_ROOT = "/cache";static Device::BuiltinAction prompt_and_wait(Device* device, int status) {status = ApplyFromAdb(device, false /* rescue_mode */, &reboot_action);} else {adb = false;
-          status = ApplyFromSdcard(device, ui);
+          status = ApplyFromSdcard(device, ui, force_ota_path);}ui->Print("\nInstall from %s completed with status %d.\n", adb ? "ADB" : "SD card", status);static void log_failure_code(ErrorCode code, const std::string& update_package)LOG(INFO) << log_content;}+static InstallResult installOtaPackage(std::string force_ota_path, Device* device)
+{
+  LOG(INFO) << "force_ota.zip found, updating ...";
+  ui->InterruptKey();
+  ui->SetBackground(RecoveryUI::NONE);
+  ui->ShowText(false);
+  return static_cast<InstallResult>(ApplyFromSdcard(device, ui, force_ota_path));
+}+static void* usb_mount_thread(void* device) {
+  const int max_mount_cnt = 10;
+  int* install_result = static_cast<int*>(malloc(sizeof(int))