Add constant time memory comparison function os_memcmp_const

Add constant time memory comparison function os_memcmp_const

This function is meant for comparing passwords or hash values where
difference in execution time could provide external observer information
about the location of the difference in the memory buffers. The return
value does not behave like os_memcmp(), i.e., os_memcmp_const() cannot
be used to sort items into a defined order. Unlike os_memcmp(),
execution time of os_memcmp_const() does not depend on the contents of
the compared memory buffers, but only on the total compared length.

Signed-off-by: Jouni Malinen <j@w1.fi>