How to script make menuconfig to automate Linux kernel build configuration?

The Linux kernel build-system provide many build targets, the best way to know about it is probably to do a make help:

Configuration targets:
  config      - Update current config utilising a line-oriented program
  nconfig         - Update current config utilising a ncurses menu based program
  menuconfig      - Update current config utilising a menu based program
  xconfig     - Update current config utilising a QT based front-end
  gconfig     - Update current config utilising a GTK based front-end
  oldconfig   - Update current config utilising a provided .config as base
  localmodconfig  - Update current config disabling modules not loaded
  localyesconfig  - Update current config converting local mods to core
  silentoldconfig - Same as oldconfig, but quietly, additionally update deps
  defconfig   - New config with default from ARCH supplied defconfig
  savedefconfig   - Save current config as ./defconfig (minimal config)
  allnoconfig     - New config where all options are answered with no
  allyesconfig    - New config where all options are accepted with yes
  allmodconfig    - New config selecting modules when possible
  alldefconfig    - New config with all symbols set to default
  randconfig      - New config with random answer to all options
  listnewconfig   - List new options
  olddefconfig    - Same as silentoldconfig but sets new symbols to their default value
  kvmconfig   - Enable additional options for guest kernel support
  tinyconfig      - Configure the tiniest possible kernel

As jimmij says in the comments, the interesting parts are in the oldconfig related targets.

Personally, I would recommend you to go for silentoldconfig (if nothing changed in the .config file or olddefconfig if you updated your .config file with a new kernel.

merge_config.sh config fragments

$ cd linux
$ git checkout v4.9
$ make x86_64_defconfig
$ grep -E 'CONFIG_(DEBUG_INFO|GDB_SCRIPTS)[= ]' .config
# CONFIG_DEBUG_INFO is not set
$ # GDB_SCRIPTS depends on CONFIG_DEBUG_INFO in lib/Kconfig.debug.
$ cat <<EOF >.config-fragment
> CONFIG_DEBUG_INFO=y
> CONFIG_GDB_SCRIPTS=y
> EOF
$ # Order is important here. Must be first base config, then fragment.
$ ./scripts/kconfig/merge_config.sh .config .config-fragment
$ grep -E 'CONFIG_(DEBUG_INFO|GDB_SCRIPTS)[= ]' .config
CONFIG_DEBUG_INFO=y
CONFIG_GDB_SCRIPTS=y

Process substitution does not work unfortunately:

./scripts/kconfig/merge_config.sh arch/x86/configs/x86_64_defconfig \
    <( printf 'CONFIG_DEBUG_INFO=y\nCONFIG_GDB_SCRIPTS=y\n' ) 

because of: https://unix.stackexchange.com/a/164109/32558

merge_config.sh is a simple front-end for the make alldefconfig target.

When cross compiling, ARCH must be exported when you run merge_config.sh, e.g.:

export ARCH=arm64
export CROSS_COMPILE=aarch64-linux-gnu-
make defconfig
./scripts/kconfig/merge_config.sh .config .config-fragment

The merged output file can be specified explicitly with the KCONFIG_CONFIG environment variable; otherwise it just overwrites .config:

KCONFIG_CONFIG=some/path/.config ./scripts/kconfig/merge_config.sh .config .config-fragment

Buildroot automates it with BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES: https://stackoverflow.com/questions/1414968/how-do-i-configure-the-linux-kernel-within-buildroot

Related: https://stackoverflow.com/questions/7505164/how-do-you-non-interactively-turn-on-features-in-a-linux-kernel-config-file