A board support package may have pieces spread out in the kernel, but the typical parts are in
arch/, and if your board requires drivers that aren't already part of the kernel, there may be some pieces in
arch/ is set up a bit differently. Arm is an interesting one: look in
arch/arm/, you'll see several cpu types and platforms there. If you look inside a cpu type, like
arch/arm/mach-at91/, you'll see lots of files for the various specific cpus as well as
board-*.c files, where board-specific peripherals are set up.