imgtool: Fix getpriv error return with private key #1405

Job	Run time
environment (esp32, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secureb...	3m 31s
environment (esp32, serialrecovery)	2m 54s
environment (esp32s2, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	3m 23s
environment (esp32s2, serialrecovery)	3m 11s
environment (esp32s3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	3m 46s
environment (esp32s3, serialrecovery)	4m 25s
environment (esp32c3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	3m 19s
environment (esp32c3, serialrecovery)	2m 55s
environment (esp32, multi-image,multi-boot, multi)	3m 21s
environment (esp32s3, multi-image,multi-boot, multi)	3m 4s
	33m 49s

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