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/apex/Android.bp b/apex/Android.bp
index bc9e084..41d2f62 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -51,6 +51,7 @@
     prebuilts: [
         "com.android.virt.init.rc",
         "microdroid_cdisk.json",
+        "microdroid_cdisk_env.json",
         "microdroid_uboot_env",
         "microdroid_bootloader",
     ],