cmdline property in bootimg can be customized for debuggable builds

This change adds product_variables.debuggable.cmdline so that the kernel
cmdline arguments can be augmented for the debuggable builds.

To support that the type of the property has changed to string array.

Bug: 181936135
Test: inspect build.ninja
Change-Id: I81b3ead5187ccb378efabb87d1d15fbdb59d8e2f
diff --git a/android/variable.go b/android/variable.go
index 5cf9aa8..9d7c1e6 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -108,6 +108,8 @@
 			Static_libs       []string
 			Whole_static_libs []string
 			Shared_libs       []string
+
+			Cmdline []string
 		}
 
 		// eng is true for -eng builds, and can be used to turn on additionaly heavyweight debugging