boot: boot_serial: Fix single slot encrypted image list #1736

Job	Run time
config (RELEASE 2,4,6 SIGNATURE)	8m 55s
config (RELEASE 2,4,6 SIGNATURE LOW)	13m 16s
config (RELEASE 2,4,6 SIGNATURE MEDIUM)	17m 22s
config (RELEASE 8,10 SIGNATURE)	6m 21s
config (RELEASE 8,10 SIGNATURE LOW)	9m 20s
config (RELEASE 8,10 SIGNATURE MEDIUM)	11m 56s
config (MINSIZEREL 2,4,6 SIGNATURE)	15m 10s
config (MINSIZEREL 2,4,6 SIGNATURE LOW)	19m 45s
config (MINSIZEREL 2,4,6 SIGNATURE MEDIUM)	23m 39s
config (MINSIZEREL 8,10 SIGNATURE)	10m 26s
config (MINSIZEREL 8,10 SIGNATURE LOW)	13m 31s
config (MINSIZEREL 8,10 SIGNATURE MEDIUM)	16m 21s
	2h 46m 2s

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