Build ramdisk's build.prop with soong

Following the same pattern as existing soong build.props, but with
minor tweaks for ramdisk such as making the partition "bootimage".

Bug: 378146476
Test: Diff'd out/target/product/vsoc_x86_64/ramdisk/system/etc/ramdisk/build.prop before and after this cl
Change-Id: I7ab2621d78c28deb73628bc455e040247a3c8031
diff --git a/scripts/gen_build_prop.py b/scripts/gen_build_prop.py
index 0b7780e..5f52d6f 100644
--- a/scripts/gen_build_prop.py
+++ b/scripts/gen_build_prop.py
@@ -608,7 +608,7 @@
         build_product_prop(args)
       case "vendor":
         build_vendor_prop(args)
-      case "system_dlkm" | "vendor_dlkm" | "odm_dlkm":
+      case "system_dlkm" | "vendor_dlkm" | "odm_dlkm" | "bootimage":
         build_prop(args, gen_build_info=False, gen_common_build_props=True, variables=[])
       case _:
         sys.exit(f"not supported partition {args.partition}")