Need a way to conditionally update submodules

Related feature: https://source.puri.sm/Librem5/use-cases/issues/25

For things like phosh we want that for e.g. mfgtool we don't sine we have all the deps in Debian. I currently one time hacked around this by removing the submodule update from the mfgtools build.

Edited Apr 10, 2019 by Heather Ellsworth

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