releasetools: Kill the use of os.getenv('AVBTOOL').

OTA tools should pick up the avbtool, as listed in dict['avb_avbtool'],
from the current PATH (plus bin/ under the dir specified via `--path`),
the same way as handling all other host tools.

Test: `m dist`
Change-Id: I3eb4d2c61979b03d9c23b2403d9a38cf052d87ea
diff --git a/tools/releasetools/add_img_to_target_files.py b/tools/releasetools/add_img_to_target_files.py
index 5132ae7..46dd416 100755
--- a/tools/releasetools/add_img_to_target_files.py
+++ b/tools/releasetools/add_img_to_target_files.py
@@ -260,7 +260,7 @@
 
   # AVB-sign the image as needed.
   if OPTIONS.info_dict.get("avb_enable") == "true":
-    avbtool = os.getenv('AVBTOOL') or OPTIONS.info_dict["avb_avbtool"]
+    avbtool = OPTIONS.info_dict["avb_avbtool"]
     part_size = OPTIONS.info_dict["dtbo_size"]
     # The AVB hash footer will be replaced if already present.
     cmd = [avbtool, "add_hash_footer", "--image", img.name,
@@ -428,7 +428,7 @@
     logger.info("%s.img already exists; not rebuilding...", name)
     return img.name
 
-  avbtool = os.getenv('AVBTOOL') or OPTIONS.info_dict["avb_avbtool"]
+  avbtool = OPTIONS.info_dict["avb_avbtool"]
   cmd = [avbtool, "make_vbmeta_image", "--output", img.name]
   common.AppendAVBSigningArgs(cmd, name)