In case you weren't aware, you can explicitly set default values:

W ??= "x"

A ?= "aval"

https://docs.yoctoproject.org/bitbake/2.0/bitbake-user-manual/bitbake-user-manual-metadata.html#setting-a-default-value

Variables can be queried as to how they are set:

$ bitbake-getvar --recipe rpi-u-boot-scr MY_VARIABLE

Also note:

The scripts are executed by /bin/sh, which may not be a bash shell but might be something such as dash.

https://docs.yoctoproject.org/bitbake/2.0/bitbake-user-manual/bitbake-user-manual-metadata.html#shell-functions