add vchiq mailbox unbind to pieeprom-update.sh #3279

Job	Run time
Yocto / Approved commit	3s
Yocto / Source mirror IAM role	5s
Yocto / Device info	51s
Yocto / Build	1h 2m 35s
Yocto / Test (cloud, balena-cloud.com, ubuntu-latest)	1h 13m 48s
Yocto / Test (hup, balena-cloud.com, ubuntu-latest)	39m 51s
Yocto / Test (os, balena-cloud.com, ubuntu-latest)	2h 5m 20s
Yocto / Deploy AMI	-1s
Yocto / Deploy to S3	-1s
Yocto / Deploy hostApp	0s
Yocto / All jobs (rt-rpi-300, balena-cloud.com)	3s
	5h 2m 34s

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