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.

.gitignore

@@ -1,3 +1,5 @@
 /images
 /linux
 /output
+/.prepared
+/.host-tools