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/libmagenta.seed

@@ -0,0 +1,12 @@
+[seed]
+name = 'libmagenta'
+
+[dependency]
+no_standard_dependencies = true
+
+[build]
+build_system = 'cmake'
+source_dir = 'kernel/libmagenta'
+watch_files = [ 'src:**/*.c', 'src:**/*.h' ]
+
+# vim: ft=toml