string: add strcasestr()

While this is not likely needed by any "real" driver code, a later
convenience addition to the "config" command will need this. As usual,
the linker will throw it away if nothing actually uses it, so it
should have no size impact when not used.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>

include/linux/string.h

@@ -71,6 +71,7 @@ extern char * strstr(const char *,const char *);
 #ifndef __HAVE_ARCH_STRNSTR
 extern char *strnstr(const char *, const char *, size_t);
 #endif
+char *strcasestr(const char *, const char *);
 #ifndef __HAVE_ARCH_STRLEN
 extern __kernel_size_t strlen(const char *);
 #endif