bootimg.cmdline is arch specific

microdroid_boot.img needs different kernel cmdline for x86_64 and arm64.

Bug: 185082754
Test: atest MicrodroidHostTests
Change-Id: I69be83cbdada6c49a033da2787908490ee76de36
diff --git a/filesystem/bootimg.go b/filesystem/bootimg.go
index 3dcc416..29a8a39 100644
--- a/filesystem/bootimg.go
+++ b/filesystem/bootimg.go
@@ -61,7 +61,7 @@
 	Vendor_boot *bool
 
 	// Optional kernel commandline
-	Cmdline *string
+	Cmdline *string `android:"arch_variant"`
 
 	// File that contains bootconfig parameters. This can be set only when `vendor_boot` is true
 	// and `header_version` is greater than or equal to 4.