string: Provide a slimmed-down memset()

string: Provide a slimmed-down memset()

Most of the time the optimised memset() is what we want. For extreme
situations such as TPL it may be too large. For example on the 'rock'
board, using a simple loop saves a useful 48 bytes. With gcc 4.9 and
the rodata bug, this patch is enough to reduce the TPL image below the
limit.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>