[python] [bug]Use actual value of generateSourceCodeOnly

[svenpanne]

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Description of the PR

The generateSourceCodeOnly was implicitly always true, regardless of its actual value. This has been fixed.

@taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @Jyhess

[rienafairefr]

Looks good to me.
I added the generateSourceCodeOnly feature, It was supposed to be a boolean flag (-DgenerateSourceCodeOnly= or -DgenerateSourceCodeOnly means generateSourceCodeOnly true, and its absence mean false). supporting -DgenerateSourceCodeOnly=<boolean_value> is good and is more consistent with other options, 👍

[kenjones-cisco]

LGTM! 👍

[wing328]

@rienafairefr @kenjones-cisco thanks for reviewing the change.\

@svenpanne thanks for your contribution.