[U-Boot] New to u-boot

Hello Guys, I am new to u-boot. I want to understand the flow of u-boot.

I have tried to go through start.s file, but I stop after arm_boot function call.

I did not understand that how board gets configured? in older version of u-boot i can see <board name>_config option in makefile, but in newer version it is not available in Makefile. So where it is?

I also want to know that during compilation how specific board files links with each other?