releasetools: Specify a minimum size for right-sized images.

On aosp_blueline, system_other gets right-sized to 61,440 bytes. This
messes up the AVB footer calculation, and is too small to store an ext4
image.

Bug: 116608795
Test: build aosp_blueline w/ dynamic partitions
Change-Id: If0138a9d858765ff82cb957532504a066fd113ff

tools/releasetools/build_image.py

@@ -431,6 +431,9 @@ def BuildImage(in_dir, prop_dict, out_file, target_out=None):
           size = common.RoundUpTo4K(size)
         else:
           size = ((size + block_size - 1) // block_size) * block_size
+        # Use a minimum size, otherwise we will fail to calculate an AVB footer
+        # or fail to construct an ext4 image.
+        size = max(size, 256 * 1024)
       extfs_inode_count = prop_dict["extfs_inode_count"]
       inodes = int(fs_dict.get("Inode count", extfs_inode_count))
       inodes -= int(fs_dict.get("Free inodes", "0"))