Fixing make-obs script being broken

[İlteriş Yağıztegin Eroğlu via Bird-users]

Heya,

I have noticed that `tools/make-obs` is broken because of the "nounset" bash 
option being enabled while directly testing an environment variable ($RELEASE) 
that's optional/not set.

I've included a simple patch to test the $RELEASE variable with bash parameter 
expansions, which will fix the script from not working.

Cheers,
ilteriş

0001-chore-make-obs-test-RELEASE-variable-with-parameter-.patch
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