where sector size (disk block size) is required, instead of
DEV_BSIZE constant which means device I/O block size.
"Looks reasonable" from dholland@, and fixes tools installboot(8)
on Cygwin where DEV_BSIZE != 512 as mentioned in PR toolchain/42555.
the secondary boot is stored to embed them into the primary bootxx)
for ext2fs.
Currently disabled because there is no primary boot block
which supports ext2fs, but it might help someone in some day.
