Fix missing build directory

Created by: andre-richter

Currently, build/bootloader is the first make target being executed during make all. However, after a clean checkout, the build directory does not yet exist. Create it accordingly.

redox@2e39ebdd49f0<redox-os-docker>:/home/arichter/repos/redox$ make
nasm -f bin -o build/bootloader -D ARCH_x86_64 -ibootloader/x86_64/ bootloader/x86_64/disk.asm
nasm: fatal: unable to open output file `build/bootloader'
mk/disk.mk:2: recipe for target 'build/bootloader' failed
make: *** [build/bootloader] Error 1