By building the posix library as shared object with an ABI, we
effectively decouple posix-using programs from the library
implementation (which happens to depend on several os-level APIs such as
the VFS).
Parse ``<env key="..." value=".."/>`` nodes from the config ROM and
populate a list at the 'genode_envp' and 'environ' symbols.
Test script at run/libc_getenv.
Fix#2236
