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/Android.bp b/Android.bp
index 9763622..1219c62 100644
--- a/Android.bp
+++ b/Android.bp
@@ -234,3 +234,12 @@
     relative_install_path: "etc", // odm_dlkm/etc/build.prop
     visibility: ["//visibility:private"],
 }
+
+build_prop {
+    name: "ramdisk-build.prop",
+    stem: "build.prop",
+    ramdisk: true,
+    product_config: ":product_config",
+    relative_install_path: "etc/ramdisk", // ramdisk/system/etc/ramdisk/build.prop
+    visibility: ["//visibility:private"],
+}