It turns out, in either local.conf or any top level conf / bb files, people can specify
- SRCREV_default_pn-${recipe name} ?= "XXXX"
Then, in the corresponding recipe bb file, just change
- SRCREV = "XXXXX"
to
- SRCREV ?= "XXXXX"
This works for both applications and the Linux kernel / U-Boot.
file: /poky/bitbake/lib/bb/fetch2/__init__.py
https://stackoverflow.com/questions/42093482/how-to-conditionally-specify-a-variable-in-yocto-bb-recipe
In addition, it is also possible to create machine based SRCREV.
