string.h

strncasecmp() is present as strnicmp() but disabled. Make it available
and define strcasecmp() also. There is a only a small performance penalty
to having strcasecmp() call strncasecmp(), so do this instead of a
standalone function, to save code space.

Update the prototype in arch-specific headers as needed to avoid warnings.

Signed-off-by: Simon Glass <sjg@chromium.org>