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.

podman-build.sh

@@ -22,7 +22,7 @@ cid=$(podman run \
 
 ./patch-uboot.sh
 
-podman exec -it ${cid} ./build-all.sh "${target}"
+podman exec -it ${cid} make
 
 podman stop ${cid}
 podman rm ${cid}