api: add macro that zeros out api reply buffer

Add macros REPLY_MACRO2_ZERO, REPLY_MACRO3_ZERO that zeros the buffer
before the values are set. This way if say, a u8[64] field is
only filled with 10 octets, the previous buffers trailing
contents are not sent.

Type: feature

Change-Id: Ia69cc9aa477f463336483f7556de32476a6f2d51
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>