libutils: Import isc_tsmemcmp() from NTP as consttime_memcmp()

This patch imports a constant-time byte sequence comparison function
from the NTP source tree [1]. Contrary to our buf_compare_ct() fun

libutils: Import isc_tsmemcmp() from NTP as consttime_memcmp()

This patch imports a constant-time byte sequence comparison function
from the NTP source tree [1]. Contrary to our buf_compare_ct() function,
which has only two possible outcomes (zero or non-zero), the new function
is a direct replacement for memcmp() because it returns three possible
states (< 0, 0, > 0).

Link: [1] http://bk.ntp.org/ntp-stable/lib/isc/tsmemcmp.c
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Suggested-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

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