zephyr: firmware/single_loader: Fix compile warning #1751

Job	Run time
config (RELEASE 2,4,6 SIGNATURE)	9m 8s
config (RELEASE 2,4,6 SIGNATURE LOW)	13m 13s
config (RELEASE 2,4,6 SIGNATURE MEDIUM)	17m 22s
config (RELEASE 8,10 SIGNATURE)	6m 38s
config (RELEASE 8,10 SIGNATURE LOW)	9m 13s
config (RELEASE 8,10 SIGNATURE MEDIUM)	11m 52s
config (MINSIZEREL 2,4,6 SIGNATURE)	14m 58s
config (MINSIZEREL 2,4,6 SIGNATURE LOW)	19m 36s
config (MINSIZEREL 2,4,6 SIGNATURE MEDIUM)	23m 33s
config (MINSIZEREL 8,10 SIGNATURE)	10m 28s
config (MINSIZEREL 8,10 SIGNATURE LOW)	13m 34s
config (MINSIZEREL 8,10 SIGNATURE MEDIUM)	16m 13s
	2h 45m 48s

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