zephyr: Fix sysflash and undefined function warning #1750

Job	Run time
config (RELEASE 2,4,6 SIGNATURE)	8m 57s
config (RELEASE 2,4,6 SIGNATURE LOW)	13m 21s
config (RELEASE 2,4,6 SIGNATURE MEDIUM)	17m 9s
config (RELEASE 8,10 SIGNATURE)	6m 22s
config (RELEASE 8,10 SIGNATURE LOW)	9m 7s
config (RELEASE 8,10 SIGNATURE MEDIUM)	11m 53s
config (MINSIZEREL 2,4,6 SIGNATURE)	14m 52s
config (MINSIZEREL 2,4,6 SIGNATURE LOW)	19m 37s
config (MINSIZEREL 2,4,6 SIGNATURE MEDIUM)	23m 34s
config (MINSIZEREL 8,10 SIGNATURE)	10m 30s
config (MINSIZEREL 8,10 SIGNATURE LOW)	13m 38s
config (MINSIZEREL 8,10 SIGNATURE MEDIUM)	16m 19s
	2h 45m 19s

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