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/img_from_target_files.py b/tools/releasetools/img_from_target_files.py
index cbb51e1..0b2b187 100755
--- a/tools/releasetools/img_from_target_files.py
+++ b/tools/releasetools/img_from_target_files.py
@@ -124,7 +124,7 @@
 
   for image_path in [name for name in namelist if name.startswith('IMAGES/')]:
     image = os.path.basename(image_path)
-    if OPTIONS.bootable_only and image not in('boot.img', 'recovery.img', 'bootloader'):
+    if OPTIONS.bootable_only and image not in('boot.img', 'recovery.img', 'bootloader', 'init_boot.img'):
       continue
     if not image.endswith('.img') and image != 'bootloader':
       continue