lib: posix: Kconfig: Allow to enable individual components

Allow to enable individual POSIX components, like Pthreads.
CONFIG_POSIX_API now just enables all of individual POSIX components,
and sets up environment suitable to easily port POSIX applications to
Zephyr.

Fixes: #12965

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
3 files changed