redbull_defconfig: Optimize for size with bare minimum functionality

We need to minimize the kernel size for use in a simple bootable
flasher.

Image.lz4 size is down from 21 MiB to 6.2 MiB after this change,
uncompressed Image from 42 MiB to 13 MiB.

Signed-off-by: Danny Lin <danny@kdrag0n.dev>