Make is compiling unneeded packages

While I compile LEDE for my target (TP-LINK WR841ND v8) with make
I can see:

make[2] package/compile
make[3] -C package/firmware/prism54-firmware compile
make[3] -C package/firmware/b43legacy-firmware compile

Why is it compiling prism54 firmware and broadcom b43 driver?
My target is Atheros, and I have set no flags about prism54.
Is it all okay? I think it should not compile this two (probably others?) packages.

I'm testing with default config for my target:

rm .config
make defconfig
make menuconfig (selected my target)

Flags about b43 and prism54 in .config:

$ cat .config | grep b43
# CONFIG_PACKAGE_b43legacy-firmware is not set
# CONFIG_PACKAGE_kmod-b43 is not set
# CONFIG_PACKAGE_kmod-b43legacy is not set

$ cat .config | grep prism
# CONFIG_PACKAGE_prism54-firmware is not set
# CONFIG_PACKAGE_kmod-net-prism54 is not set

I recall something about buildroot and that buildroot devs decided not to do the extra work to have the build automatically figure out what needs to be rebuilt.

I always think it's safer to do make clean or make clean all. I always keep a copy of .config in some place out of the build tree. But I think only make distclean removes .config files.