app-portage/prefix-toolkit: new package

app-portage/prefix-toolkit: new package

* For any Gentoo Prefix, provides the 'startprefix' script, and should
provide some 'runprefix' script later on, as inspired in
Bug: https://bugs.gentoo.org/673816

* For base Gentoo Prefix (as in "not stacked"), provides the
'prefix-stack-setup' script, superseding app-portage/prefix-chain-setup,
does force the prefix-stack implit USE flag in the stacked Prefix.
* For stacked Gentoo Prefix (set up using 'prefix-stack-setup'),
provides the toolchain wrapper, superseding sys-apps/prefix-chain-utils.
Bug: https://bugs.gentoo.org/658572

Closes: https://bugs.gentoo.org/509142
Package-Manager: Portage-2.3.51, Repoman-2.3.11
Signed-off-by: Michael Haubenwallner <haubi@gentoo.org>