How to print out the MB1 log?

The development guide shows a MB1 sample log, but I can’t get that messages through a serial console.

Sample Boot Log
The boot slot is selected by MB1. The following sample log shows that Slot 1 is selected as the boot slot. The message displayed from MB1 is available from a debug build of MB1 that has messages enabled. Messages from MB2 are always available.
[0000.040] I> Welcome to MB1-coldboot(dev-version : 14.00.170822-t186-M-00.00-96e265a3)
[0000.048] I> rst_source : 0xb
[0000.051] I> rst_level : 0x1
[0000.053] I> Read lock all AES keyslots
[0000.057] I> Read lock all RSA keyslots



We didn’t enable the MB1 debug log for public release. What is your purpose for enabling it?

Just want to fully understand the boot flow.