release: factor out common code from make-memstick.sh

[bz]

Summary

Move make-memstick.sh to scripts to provide a common framework.
It seemed that all four implementations were copied from one or
another while mostly being the same.

The only differences really are the layout of the created disk image
(with or without a possible ESP) which is now provided as a target
architecture specific function while the common code is simply included.

Sponsored by: The FreeBSD Foundation

Test Plan

NOT FOR 14.2; so low prio.

[emaste]

I think we can reduce some duplication in memstick_mkimg later, but this is a great start

[bz]

Thanks for the review; this was based on an earlier attempt to look at release (April this year?) for firmware and trying to figure out how things are put together.
I'll let it sit until at least RC1 is out in case other changes need to be done to avoid conflicts.

[emaste]

Yeah I'd hold off on this until after 14.2 is out the door

[jrtc27]

release/amd64/make-memstick.sh

6–7

These lines are a bit odd

release/scripts/make-memstick.sh

32

I mean, yeah, it is in practice, but can we just pass in a string representing which script we're running (i.e. what MACHINE is)?