ctk-curses: Introduce CTK-specific color names

Since ncurses also defines COLOR_BLACK and friends, and we want
to avoid including curses.h in contiki-conf.h, define CTK_COLOR_*
constants and map them to curses colors in ctk-curses.c.
ctk-curses: Revert "ctk-curses: fix build"

This reverts commit 9ffaa964753f87e23c7f31d178e0d0639f39ab2c.

Since we do not include curses.h anymore there shouldn't be any name clash.