boot: boot_serial: Fix single slot encrypted image list #1739

Job	Run time
config (RELEASE 2,4,6 SIGNATURE)	9m 0s
config (RELEASE 2,4,6 SIGNATURE LOW)	13m 21s
config (RELEASE 2,4,6 SIGNATURE MEDIUM)	17m 14s
config (RELEASE 8,10 SIGNATURE)	6m 28s
config (RELEASE 8,10 SIGNATURE LOW)	9m 16s
config (RELEASE 8,10 SIGNATURE MEDIUM)	11m 57s
config (MINSIZEREL 2,4,6 SIGNATURE)	15m 0s
config (MINSIZEREL 2,4,6 SIGNATURE LOW)	19m 42s
config (MINSIZEREL 2,4,6 SIGNATURE MEDIUM)	23m 52s
config (MINSIZEREL 8,10 SIGNATURE)	10m 29s
config (MINSIZEREL 8,10 SIGNATURE LOW)	13m 42s
config (MINSIZEREL 8,10 SIGNATURE MEDIUM)	16m 16s
	2h 46m 17s

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