imgtool: Fix getpriv error return with private key #1404

Job	Run time
environment (esp32, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secureb...	4m 8s
environment (esp32, serialrecovery)	3m 43s
environment (esp32s2, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	3m 58s
environment (esp32s2, serialrecovery)	3m 17s
environment (esp32s3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	3m 48s
environment (esp32s3, serialrecovery)	3m 49s
environment (esp32c3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	3m 39s
environment (esp32c3, serialrecovery)	3m 49s
environment (esp32, multi-image,multi-boot, multi)	3m 8s
environment (esp32s3, multi-image,multi-boot, multi)	3m 4s
	36m 23s

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