sim: Make TLV size for ECDSA signature constant

ECDSA signatures are encoded as ASN.1 and the size of the ASN.1
representation can vary depending on the value of the two integers the
signature is composed of. This means that when ECDSA is used, the size
of the TLV area is not always equal to the size that was estimated by
the simulator when attempting to determine the maximum image size.
Indeed, the estimate gives the maximum possible size of the TLV area and
depending on its actual size, the generated images might be in fact a
bit smaller than expected.

This is not a big issue but adds a bit of randomness in the simulation
and make difficult to generate precisely oversized images when desired
for example.

This commit ensures an ECDSA signature with the largest possible size is
always used, making the size of the corresponding TLV entry constant in
the simulator.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>