Move uboot_env.img to a new composite disk

u-boot is updated to look for the environment partition in the second
disk.

Bug: 183006588
Test: atest MicrodroidHostTestCases
Change-Id: I2e716bb85d4762f700ea7479c68ffbcfd150857c
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index 8456591..150f119 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -338,3 +338,8 @@
     name: "microdroid_cdisk.json",
     src: "microdroid_cdisk.json",
 }
+
+prebuilt_etc {
+    name: "microdroid_cdisk_env.json",
+    src: "microdroid_cdisk_env.json",
+}