[Buildroot] [PATCH 0 of 6 v4] infra: fix dollar signs; remove some undefined versions

[Thomas De Schampheleire]

This patch series fixes the double sign problem in the inner-xxx-targets
(see description in the patch itself), and makes the following semi-related
changes:
- convert toolchain and toolchain-buildroot package to virtual-package
- change makedevs and mkpasswd to have a 'buildroot' version

The changes are related in the sense that the double dollar signs were
needed to correctly implement the subsequent patches.

I think that the series is ready to be merged now, with Reviews/Acks on all
patches.

As follow-up I plan to do something about the toolchain-external version
which is still 'undefined', and add some extra comments in pkg-generic.mk.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire at gmail.com>

---
v4: rebase, remove change to toolchain-external from this series
v3: rebase and implement comments on first patch
v2: increase consistency of rules (first two patches)