DO NOT MERGE: fec: add --padding

Add an option to insert padding between ECC data and fec_header to
make it easier to position the header at the end of the partition.

Bug: 28865197
Bug: 32789520
Change-Id: I8c11401df56d15362fe3eb3c2a6a91ea82b02a83

(cherrypicked from commit 85e592cb86c9cbca82852240f0148adbee1b74e8)
3 files changed