Add bootimg module type

The new module type is to build the boot partition images. Boot
partition image consists of kernel, ramdisk, dtb, etc. which are
processed by the mkbootimg tool. There are two flavors of boot
partition: boot and vendor_boot. The choice is configured via the
'vendor_boot' property. Currently, only `vendor_boot: true` case is
supported.

But: 178978059
Test: m cf_bootimg-5.10
Change-Id: Iee689d9d666e8e6e5af7db02a76dd39d1e4d0e3a
diff --git a/filesystem/Android.bp b/filesystem/Android.bp
index 926df6e..9994241 100644
--- a/filesystem/Android.bp
+++ b/filesystem/Android.bp
@@ -7,6 +7,7 @@
         "soong-android",
     ],
     srcs: [
+        "bootimg.go",
         "filesystem.go",
     ],
     testSrcs: [