bootutil: Fix signed/unsigned comparison in boot_read_enc_key #1824

Job	Run time
environment (esp32, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secureb...	1m 40s
environment (esp32, serialrecovery)	1m 36s
environment (esp32s2, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	1m 40s
environment (esp32s2, serialrecovery)	1m 21s
environment (esp32s3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	1m 42s
environment (esp32s3, serialrecovery)	1m 22s
environment (esp32c3, secureboot-sign-rsa2048,secureboot-sign-rsa3072,secureboot-sign-ec256,secur...	1m 48s
environment (esp32c3, serialrecovery)	1m 27s
environment (esp32, multi-image,multi-boot, multi)	1m 39s
environment (esp32s3, multi-image,multi-boot, multi)	1m 33s
	15m 48s

Provide feedback