Defer evaluation of modified until after it's set

[imp]

Summary

With the reorg r348175, we now look at modified before it is
set. Rearrange things so that we can set include_metadata to either
yes, no or some variable to be evaluated in the future (in this case
modified). Use the eval echo trick to expanded it. This should fix the
-R flag that was broken in r348175, which broke
WITH_REPRODUCIBLE_BUILD for kernels.

[emaste]

No objection but I'm generally not a fan of the deferred evaluation; what about just setting include_metadata=yes/no/if-modified and making the test something like
if [${include_metadata} = yes || (${include_metadata} = if-modified && ${modified} = yes)]

Anyway if you prefer this approach that's fine with me

[imp]

Fix per ed (though a little different construct)
Fix backwards test after converting from -z notation...

[imp]

In D20480#442157, @emaste wrote:

No objection but I'm generally not a fan of the deferred evaluation; what about just setting include_metadata=yes/no/if-modified and making the test something like
if [${include_metadata} = yes || (${include_metadata} = if-modified && ${modified} = yes)]

Anyway if you prefer this approach that's fine with me

Due to the funky way that test works, I opted for a slightly different way to say this...