Recent versions of libfdt add a dependency on strnlen. Copy the
implementation in lib/string.c here, so we can update libfdt.
Acked-by: NRussell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: NRob Herring <robh@kernel.org>

This is a small subset of string functions needed by commits to come.
Except for memcpy() which is unchanged from its original location, their
implementation is meant to be small, and -Os is enforced to prevent gcc
from doing pointless loop unrolling.
Signed-off-by: NNicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: NShawn Guo <shawn.guo@linaro.org>
Tested-by: NDave Martin <dave.martin@linaro.org>
Tested-by: NThomas Abraham <thomas.abraham@linaro.org>