scripts/imgtool.py: Add PKCS#11 ECDSA P384 support#2444

IanJ-Arm wants to merge 1 commit intomcu-tools:mainmcu-tools/mcuboot:mainfrom

IanJ-Arm:imgtool-pkcs11-signing-flatIanJ-Arm/mcuboot:imgtool-pkcs11-signing-flatCopy head branch name to clipboard