sys/boot: always use freshly build libstand

[allanjude]

Summary

In some cases sys/boot was using the system /usr/lib/libstand.a instead of the one we just built

[allanjude]

@imp I know you are working on a sys/boot cleanup, so I figured you might want to integrate this

[imp]

Yea, don't do this.

(a) It conflicts with what I'm doing trying to disentangle things (eg it creates another mess I have to disentangle)
(b) we don't do this anyplace else
(c) It introduces yet more horror of cut and paste, which is killing us by a thousand cuts.
(d) libstand really shouldn't be under lib, but should be sys/boot/libstand instead. If it were, then the normal usage patterns would obviate the need for 90% of this patch.