Fix bash env var conditionals in merge.py call to be consistent

Fix bash env var conditionals in merge.py call to be consistent

Bash syntax ${ENV:-'0'} and ${ENV='0'} vary only in their behavior
when $ENV is set to an empty string (in which case the former
expression results in '0' while the latter results in ''.)

There is no reason the Makefile should be using the latter
expression, and rather confusing when the two different forms
are used side-by-side on the same line.

Change-Id: I66bb5ea7437c3f2aa9bc4e1afc9b522c450ff976