boot_serial: Fix include #1647

Job	Run time
config (RELEASE 2,4,6 SIGNATURE)	9m 33s
config (RELEASE 2,4,6 SIGNATURE LOW)	13m 58s
config (RELEASE 2,4,6 SIGNATURE MEDIUM)	18m 53s
config (RELEASE 8,10 SIGNATURE)	6m 53s
config (RELEASE 8,10 SIGNATURE LOW)	9m 50s
config (RELEASE 8,10 SIGNATURE MEDIUM)	12m 46s
config (MINSIZEREL 2,4,6 SIGNATURE)	15m 32s
config (MINSIZEREL 2,4,6 SIGNATURE LOW)	22m 12s
config (MINSIZEREL 2,4,6 SIGNATURE MEDIUM)	24m 47s
config (MINSIZEREL 8,10 SIGNATURE)	10m 51s
config (MINSIZEREL 8,10 SIGNATURE LOW)	14m 9s
config (MINSIZEREL 8,10 SIGNATURE MEDIUM)	17m 9s
	2h 56m 33s

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