zephyr: Add estimated size of update trailer to sysbuild #1793

Job	Run time
config (RELEASE 2,4,6 SIGNATURE)	9m 4s
config (RELEASE 2,4,6 SIGNATURE LOW)	13m 25s
config (RELEASE 2,4,6 SIGNATURE MEDIUM)	17m 7s
config (RELEASE 8,10 SIGNATURE)	6m 22s
config (RELEASE 8,10 SIGNATURE LOW)	9m 17s
config (RELEASE 8,10 SIGNATURE MEDIUM)	12m 31s
config (MINSIZEREL 2,4,6 SIGNATURE)	14m 57s
config (MINSIZEREL 2,4,6 SIGNATURE LOW)	19m 41s
config (MINSIZEREL 2,4,6 SIGNATURE MEDIUM)	23m 50s
config (MINSIZEREL 8,10 SIGNATURE)	10m 37s
config (MINSIZEREL 8,10 SIGNATURE LOW)	13m 29s
config (MINSIZEREL 8,10 SIGNATURE MEDIUM)	16m 15s
	2h 46m 35s

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