Make sure MACHINE(_ARCH) are exported for depend-cleanup.sh

[jrtc27]

Summary

We run depend-cleanup.sh twice during the build. The second time is the
normal run, where we run it under WMAKEENV and thus have CROSSENV's
MACHINE(_ARCH)=${TARGET(_ARCH)} in the environment. However, the first
time is for bootstrap-tools, where it's run under BMAKEENV and we don't
have any assignments to MACHINE(_ARCH) in the environment, meaning the
script sees them as unset. In practice this doesn't matter since the
only use doesn't apply to bootstrap-tools, but it could be a future
issue. Thus, explicitly export them for depend-cleanup.sh and have the
script verify they're set.

[imp]

Makes sense

[sjg]

tools/build/depend-cleanup.sh

27

FWIW I'd be inclinded to add an error() function - hate repeating logic

test -z "$MACHINE" && error MACHINE not set

etc

[jrtc27]

tools/build/depend-cleanup.sh

27

I would have done except that D31007 already does that and I didn't feel the need to reorganise things just for that