Add top-level Makefile

Building the OS is now as simple as running `make` on a Gentoo system.

Interestingly, when `make` is executed as a (grand)child process of
another `make` process, it always prints an `Entering directory ...`
message.  This breaks the `make kernelversion` command, by adding
extraneous text to the output.

build-kernel.sh

@@ -15,11 +15,11 @@ cd linux
     ../linux.config
 make -C /usr/src/linux O=${PWD} olddefconfig
 make -j$(nproc)
-kver=$(make kernelversion)
+kver=$(make -s kernelversion)
 make modules_install INSTALL_MOD_PATH=/mnt/gentoo
 cd -
 
-printf 'Installing Kernel ...\n'
+printf 'Installing Kernel %s ...\n' "${kver}"
 mkdir -p /mnt/gentoo/boot
 cp -au linux/arch/arm64/boot/Image.gz /mnt/gentoo/boot/vmlinuz-${kver}
 cp -au linux/.config /mnt/gentoo/boot/config-${kver}