buildroot

busybox: unify duplicated build steps

This does set a few more vars in some steps that weren't set before, but
they should be harmless in the contexts in question.  And the resulting
save in duplication is nice.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>