updater: Add support for FKM OTA kernel updates

Link for users:
https://raw.githubusercontent.com/Zile995/android_kernel_xiaomi_msm8953/oreo-mr1/updater/update.json

Signed-off-by: Zile995 <stefan.zivkovic995@gmail.com>

updater/changelog

@@ -0,0 +1,12 @@
+**Kernel: Underground Kernel**
+
+Device: Redmi Note 4(X) Snapdragon (mido)
+Compiler: AOSP Clang **11.0.3**
+
+- Latest Linux kernel updates from android-3.18 common tree (synced with sources up to **July 16**, 2020).
+- Fixed FKM OTA
+- **If you want to change to another custom kernel on android 10:**
+  - **You have to flash Underground-uninstaller.zip and stock boot.img (from your ROM.zip)**
+  - After that you can flash Magisk and other custom kernel
+- Do not use FKM OTA updates over the other custom kernels!
+- You can use it with stock or Underground kernel only!

updater/update.json

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+{
+  "kernel": {
+    "name": "Underground Kernel",
+    "version": "r57",
+    "link": "https://github.com/Zile995/android_kernel_xiaomi_msm8953/releases/download/r57/UndergroundKernel-mido-20200717.zip",
+    "changelog_url": "https://raw.githubusercontent.com/Zile995/android_kernel_xiaomi_msm8953/oreo-mr1/updater/changelog",
+    "date": "2020-07-17",
+    "sha1": "462cf8ff4c8a72494f2898fd9acaf1f05daf9b5b"
+  },
+  "support": {
+    "link": "https://t.me/rn4official"
+  }
+}
+