Add init_first_stage to riscv64

aosp_riscv64 doesn't use base-system.mk yet, add init_first_stage
to the ramdisk manually.

Test: lunch aosp_riscv64-userdebug && m out/target/product/generic_riscv64/ramdisk.img
Change-Id: I148e436b34e8900815eb8dac42a6c1f5251f824d
diff --git a/target/product/aosp_riscv64.mk b/target/product/aosp_riscv64.mk
index 023317b..436ff97 100644
--- a/target/product/aosp_riscv64.mk
+++ b/target/product/aosp_riscv64.mk
@@ -62,6 +62,7 @@
 # just the modules that work for riscv64.
 PRODUCT_PACKAGES := \
   init.environ.rc \
+  init_first_stage \
   init_system \
   linker \
   shell_and_utilities \