build.sh flag "-u"

[Arthur Barlow <arthurbarlow%gmail.com@localhost>]

I have some uncertainty about the application of the "-u" flag in
build.sh.  The docs say it prevents "make clean."  But without it will
rebuild everything, including tools.  I've used the flag before
without problems, but recently I've had the build break on a couple of
occasions.  I'm not doing any cross compile and I'm building amd64.

When I delete all the files in /usr/obj, (my obj directory), and don't
use the "-u" flag, everything seems to build fine.  Can anyone provide
any wisdom on whether to use or not use this flag?