Files in $element/environment.d are meant to be sourced, so drop
the executable bit. Moreover, drop the executable bit from a couple
of other scripts that are either meant to be sourced or simply because
they are configuration files.
Change-Id: I7f724dd9d409f4a835a136f12f48a84aa9acc41e
Optionally remove portage files, so that we can cache package and
keep the portage directory around, specifically for nodepool.
This also adds a section to the Gentoo readme about the variable
and renames the 00-gentoo-distro-name environment file to a more
appropriate name of 00-gentoo-envars.
Also brought up was the location of the gentoo-releng.gpg file,
this has been moved and the refrencing paths updated.
Change-Id: I20c91b36082828faa1ca481585acc5f9933211e1
