u-boot-update: Make 'prompt' configurable

Diederik de Haasrequested to merge
diederik/u-boot-menu:make-prompt-value-configurable into debian/master

The 'prompt' value was hard-coded to '0', which means the menu isn't actually shown and there was no non-hacky way to change that.
So replace the hard-coded '0' with an U_BOOT_PROMPT variable to make it configurable.

Closes: #1034071

Edited by Diederik de Haas

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