I am using a Linux-v4.9.212. For a better linux kernel boot process debug, I would like to view all possible messages during the linux kernel boot. I have tried to set 2 different parameters in the "chosen" part of the device tree (.dtb) for that debug purpose but it has not brought about a printout of any additional messages. The parameters I used are "debug" and "initcall_debug". Please advise on the syntax of using these parameters and whether there could be another issue that could potentially disable the additional printout of the messages even though the parameters have been used correctly.

Probably you might be looking for this symbol:
CONSOLE_LOGLEVEL_DEFAULT

kernel-hacking -> printk and dmesg options..

But there are other things than only that as well..