Use defconfig to generate .config

Use minimal set of config selections and let coreboot generate the
default values for the rest of them.

The only differences are the following models selecting
CONFIG_CPU_MICROCODE_CBFS_DEFAULT_BINS instead of *_EXTERNAL_BINS:

- darp5
- darp6
- galp3-c
- galp4
- lemp9

scripts/_build/coreboot.sh

@@ -16,7 +16,7 @@ pushd coreboot >/dev/null
     make CPUS="$(nproc)" crossgcc-i386
   fi
   make distclean
-  cp -v "${CONFIG}" .config
+  make defconfig KBUILD_DEFCONFIG="${CONFIG}"
   make --jobs="$(nproc)"
   cp -v "build/coreboot.rom" "${COREBOOT}"
 popd >/dev/null