fix(app-shells/bash): Update /etc symlinks, use /usr/share/skel

Symlinks out of /etc to /usr should generally be relative, that way they
work regardless of whether you are based at / or dealing with a new
chroot you haven't entered yet, or a build root like /build/amd64-usr
Namely the absolute links break cros_sdk which copies skel into home.

Also switch to /usr/share/skel since other packages may need install
things to that directory in the future.
Merge pull request #353 from marineam/etc-symlinks

fix(app-shells/bash): Update /etc symlinks, use /usr/share/skel