Buildroot shouldn't be compared with a full blown Linux distribution. You need to take care of preparing your filesystem manually.
/etc/modules is a configuration file, but without a startup script, it is useless.
You can prepare
/etc/modules file content as Raspbian and create an init script for buildroot to parse this file and load the required modules.
It will be good to have an understanding about init systems in Linux generally. In the good old days of init systems, there was only sysv init. But now we have more options and buildroot already supports at least three of them:
- busybox init
- sysv init
You can read more about here:
As an example, if you're using busybox init in buildroot, you can create a startup script like
/etc/init.d/S01-modules and parse /etc/modules in here to load modules with modprobe or insmod.