kernel: Use LIBDIR var in Makefile
kernel: Use LIBDIR var in Makefile

Instead of using a hard-coded path, use the LIBDIR variable so
the library path can be overridden.

veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi