zephyr: Use string.h instead of EC definitions
zephyr: Use string.h instead of EC definitions

The EC does not have a string.h header but instead puts these functions
in the util.h header. For zephyr it is better that we use the C
library, so update these.

Leave along those which are specific to ECOS.

BUG=b:180023514
BRANCH=none
TEST=build zephyr for volteer

Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Change-Id: I03421e41a098501c3eb0f651c0960beab8e7428a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2691412
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec