lib: add glibc style strchrnul() variant

The strchrnul() variant helpfully returns a the end of the string
instead of a NULL if the requested character is not found. This can
simplify string parsing code since it doesn't need to expicitly check
for a NULL return. If a valid string pointer is passed in, then a valid
null terminated string will always come back out.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Change-Id: Iad6409d01a9891d31e5447509aa2ed8d59f7a874