Environment variables with `lib/posix-environ`

Environment variables with `lib/posix-environ`

Commit 980bf211 enabled code for managing environment variables with musl,
however they were never populated during boot. With PR ???, `posix-environ`
envrionment variables are officially supported by Unikraft. This commit
enables and integrates this implementation with musl.

Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Delia Pavel <delia_maria.pavel@stud.acs.upb.ro>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #43