nxd-bom: First draft

Program nxd-bom is intended to help generating bill of material for a
software-release or slapos node.

Generation of the BOM for a software-release is already draftly
implemented. Generation of the BOM for slapos node is thought-out, but
not currently implemented.

In provided example/ors-bom.txt it could be seen how both parts should
look like. And nxd-bom documents in corresponding TODO section how
BOM(slapos-node) should be computed.

I originally used this draft script to build ors-bom.txt as requested by
a customer, and I no longer actively work on it. However I suggest we
incrementally improve it as needed and maybe also integrate it to be run
automatically in _all_ our builds, so that whenever an SR is built, the
BOM.txt is also built automatically for it.