Added a top level `posix` shell command for other POSIX
commands. Currently only `uname` is supported.
New POSIX commands can be added by including the
`posix_shell.h` header and use the `POSIX_CMD_ADD` helper
macro.
Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Relocate the `uname` shell implementation from uname sample, so
that it can be reused by other application and the uname sample
only uses POSIX APIs.
Signed-off-by: Yong Cong Sin <ycsin@meta.com>