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

filesystem/bootimg.go

@@ -60,8 +60,8 @@ type bootimgProperties struct {
 	// https://source.android.com/devices/bootloader/partitions/vendor-boot-partitions
 	Vendor_boot *bool
 
-	// Optional kernel commandline
-	Cmdline *string `android:"arch_variant"`
+	// Optional kernel commandline arguments
+	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.
@@ -152,7 +152,7 @@ func (b *bootimg) buildBootImage(ctx android.ModuleContext, vendor bool) android
 	dtb := android.PathForModuleSrc(ctx, dtbName)
 	cmd.FlagWithInput("--dtb ", dtb)
 
-	cmdline := proptools.String(b.properties.Cmdline)
+	cmdline := strings.Join(b.properties.Cmdline, " ")
 	if cmdline != "" {
 		flag := "--cmdline "
 		if vendor {