bootutil/crypto: Add license disclaimer to ecdsa_p256.h

The ecdsa abstraction layer header does not have a license header.
Add it in preparation for the merge with ecdsa.h

Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>
Change-Id: Ibb906c7f6bb3b50387e4816e1a95b31d3c8b515f