scripts/jlib.py: some minor additions
scripts/jlib.py: some minor additions

In jlib.log() , added support for output name=value of multiple values using
'=' at start of {...} followed by space-separated names.

Improved jlib.build(): allow <infiles> and <outfiles> to include a filter
function. Fixed handling of .cmd file - previously we were always touching it,
which could lead to unnecessary rebuilds.

update_file(): simplified and added optional <return_different> arg.

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