17.1.3.1. About Buildroot’s kernel configs
To see Buildroot’s base configs, start from buildroot/configs/qemu_x86_64_defconfig.
That file contains BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="board/qemu/x86_64/linux-4.15.config", which points to the base config file used: board/qemu/x86_64/linux-4.15.config.
arm, on the other hand, uses buildroot/configs/qemu_arm_vexpress_defconfig, which contains BR2_LINUX_KERNEL_DEFCONFIG="vexpress", and therefore just does a make vexpress_defconfig, and gets its config from the Linux kernel tree itself.