meta: replace cmake mono-build system with a custom build co-ordinator

every system component now has its own self-contained build system, and a
seed file describing the component and its build system and dependencies.

a new tool, meadow, collects and uses these seed files to build the
components into a full system. meadow also manages the target system root
and a host prefix where toolchain tools are installed to.

the build system has also been updated to use a new $ARCH-rosetta-gcc
compiler, and the patches files necessary to build it have been
added to toolchain/cross-compiler.

external/magenta-headers.seed

@@ -0,0 +1,22 @@
+[seed]
+name = 'magenta-headers'
+
+[dependency]
+no_standard_dependencies = true
+
+[build]
+build_system = 'source-only'
+build_for = 'target'
+source_dir = 'kernel/libmagenta'
+watch_files = [
+	'src:include/magenta/*.h',
+	'src:include-user/magenta/*.h',
+]
+
+[install]
+"/usr/include/magenta" = [
+	'src:include/magenta/*.h',
+	'src:include-user/magenta/*.h',
+]
+
+# vim: ft=toml