chroot-functions.sh: Fix a mis-worded comment

Fixes: 900554b0b067e76d3806e520357d91adbfa4fa0c
Author: Jorge Manuel B. S. Vicetto (jmbsvicetto) <jmbsvicetto@gentoo.org> (Mon 19 Dec 2011 11:14:05 PM PST)
subject: Make sure portage updates itself at the start of stage1 - thanks to Zac for noticing the issue.
setup_pkgmgr(): Make the 'build' use flag passed in

The "build" USE flag is only needed for the stage1 build.
It also causes other errors in later stages.
This makes setup_pkgmgr() takes an optional USE flag string
parameter to be added to the USE variable.