add note about bash join_by function

add example upgrading script

add .build.yml

fix duplicate package upgrade lines

it is common practice to have multiple kernel-* packages installed to
have a fallback version. while upgrading, we only need to 'upgrade'
_one_ of the installed packages of the same name.

nb: it may be worth a thought to rewrite the whole package upgrading, but
it is fast enough for now, so that the double loop isn't too painful for
now.

go modulize

add debug flag

remove check for uid/gid in FILELIST.TXT

initial commit