Update releasetools for new init_boot.img

This new init_boot.img contains the ramdisk that used to reside in the
boot.img file.

Test: set BOARD_PREBUILT_INIT_BOOT_IMAGE to an external init_boot.img
      - Check that "m" pulls in the init_boot.img to
      out/target/product/vsoc_x86_64/
      - Check that "m dist" adds the init_boot.img to
      aosp_cf_x86_64_phone-img-eng.devinmoore.zip
Test: atest --host releasetools_test
Bug: 203698939
Change-Id: If7ef2cf093e5e525529c7c44333c0f40f6ba0764
diff --git a/tools/releasetools/sign_target_files_apks.py b/tools/releasetools/sign_target_files_apks.py
index 5626980..8b837b8 100755
--- a/tools/releasetools/sign_target_files_apks.py
+++ b/tools/releasetools/sign_target_files_apks.py
@@ -201,6 +201,7 @@
 
 AVB_FOOTER_ARGS_BY_PARTITION = {
     'boot': 'avb_boot_add_hash_footer_args',
+    'init_boot': 'avb_init_boot_add_hash_footer_args',
     'dtbo': 'avb_dtbo_add_hash_footer_args',
     'product': 'avb_product_add_hashtree_footer_args',
     'recovery': 'avb_recovery_add_hash_footer_args',