Replace deprecated mbedtls_sha256 with mbedtls_sha256_ret
The mbedtls_pk_parse_key does not work correctly anymore
with the DER buffer embedded in the secure image extentson
using the buffer size as the the key length.
Move to mbedtls_pk_parse_subpubkey API that handles such
case correctly.
The DER format already contains the key length, so there
is no particular reason to supply it to the key parser.
Update the doimage version to 3.3

Change-Id: I0ec5ee84b7d1505b43138e0b7a6bdba44a6702b6
Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>