Newer versions of Coverity use built-in models for wcrtomb() and
hmac_init() that are capable of returning errors, and reports defects
due to code failing to check for these errors.  The actual iPXE
implementations are simpler than Coverity's models and can never
return errors, so these defects are false positives.

Fix by overriding Coverity's built-in models for these functions.

Signed-off-by: Michael Brown <mcb30@ipxe.org>