copy-builtin: add hooks with sed/>>

The order in fs/Makefile doesn't matter,
the order in fs/Kconfig is preserved (ext2 is included as the first
thing in the first if BUILD block, and only once), but I don't think it
matters much either, realistically

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13316