build_image: hard code set inode size to 256

inode-size will jump around based on filesystem size, however
readonly Android system partitions have a specific pattern of
xattr associated with sepolicy and 256 is the most efficient at
absorbing the content.

Test: manual
Bug: 122328872
Change-Id: I06dd6a503067ab6477293b386d56a89dd86b0e83