boot: encrypted: fix HKDF input size macro
boot: encrypted: fix HKDF input size macro

Use correct macro for the size of a DH shared secret. The previously
used macro had the same size (SHA-256 digest length) but incorrect
semantics.

Signed-off-by: Fabio Utzig <66676b1ceaf93296e098708c09494361103aa635@apache.org>

runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot,utzig/mcuboot,tamban01/mcuboot,utzig/mcuboot,ATmobica/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,tamban01/mcuboot