cleanup libc kconfig and use choice

This will allow adding more libc implementations in the future.
Also use the location of the include directory from the toolchain
settings.

Change-Id: Idae5e4ffd9b1bcb5e924da156df56e81f10c8842
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
3 files changed