[shell] make shell really optional in Linux

[gjc13]

Problem

Fix the TODO in #7205

Change overview

Add global chip_shell gn args and use it as the only flag to enable/disable shell.

Testing

Build with gn gen out/debug --args='chip_shell=false' and gn gen out/debug --args='chip_shell=true'

[andy31415]

Why do we want shell to be optional on linux?

Case for not having it optional: less config options, less likely for a build path to become stale and unmaintained.
If we do not have a sepecific use case for each of "with shell" and "without shell" it seems easier to just enable it always.