build-target-bins.sh: use ec-p256 key for signing images

TF-M BL2 switched to using EC-P256 key with PSA Crypto backend [1]. So
use EC-P256 key for signing the images.

[1]: https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/29991



Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>