raspberrypi: allow directly specifying the original boot .S file

.. all the necessary steps to transform it into a padded, checksummed
file are now done by the build system.

Since it is assigned by "?=", it _should_ be the case that individual
builds can override it.

I did not "test" this per se, but it gives the same content and checksum
(except for the identifying comment with a path) as #4302.

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