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.

toolchain/meadow/dependency.py

@@ -0,0 +1,156 @@
+import os
+
+
+class DependencyResolutionError(Exception):
+    pass
+
+
+class DependencyItem:
+    def __init__(self):
+        self.__target = None
+        self.__dep_list = None
+
+
+    def name(self):
+        return None
+
+
+    def target(self):
+        return self.__target
+
+
+    def dependencies(self):
+        return None
+
+
+    def dependency_list(self):
+        return self.__dep_list
+
+
+    def publish(self):
+        return None
+
+
+    def reset_resolution(self):
+        self.__dep_list = None
+
+
+    def resolve(self, **kwargs):
+        if self.__dep_list is not None:
+            return self.__dep_list
+
+        ctx = None
+        dep_list = None
+        if 'ctx' in kwargs:
+            ctx = kwargs['ctx']
+        else:
+            ctx = {}
+
+        if 'dep_list' in kwargs:
+            dep_list = kwargs['dep_list']
+        else:
+            dep_list = []
+
+        if self.name() in ctx:
+            return
+
+        ctx[self.name()] = self
+
+        deps = self.dependencies()
+        if deps is None:
+            return
+
+        for d in deps.values():
+            d_value = d.resolve_self()
+            if d_value is not None:
+                d_value.resolve(ctx=ctx, dep_list=dep_list)
+
+        dep_list.append(self)
+        return dep_list
+
+
+class Dependency:
+    def __init__(self, parent):
+        self.__parent = parent
+
+
+    def name(self, parent):
+        return None
+
+
+    def parent_name(self):
+        return self.__parent
+
+
+    def resolve_self(self):
+        return None
+
+
+    def is_resolved(self):
+        return False
+
+
+class SeedDependency(Dependency):
+    def __init__(self, **kwargs):
+        self.__target_seed = None
+        if 'name' in kwargs:
+            self.__name = kwargs['name']
+        if 'parent' in kwargs:
+            super().__init__(kwargs['parent'])
+
+
+    def name(self):
+        return self.__name
+
+
+    def target(self):
+        return self.__target_seed
+
+
+    def resolve_self(self):
+        from seed import Seed
+        if self.__target_seed is not None:
+            return self.__target_seed
+
+        target = Seed.get(self.__name)
+        if target is None:
+            raise DependencyResolutionError(
+                    '{} depends on seed "{}", but the required seed cannot be found'.format(self.parent_name(), self.__name))
+
+        self.__target_seed = target
+        return self.__target_seed
+
+
+    def is_resolved(self):
+        return self.__target_seed is not None
+
+
+class ProgramDependency(Dependency):
+    def __init__(self, **kwargs):
+        if 'name' in kwargs:
+            self.__name = kwargs['name']
+        if 'parent' in kwargs:
+            super().__init__(kwargs['parent'])
+
+
+    def __is_exe(self, fpath):
+        return os.path.isfile(fpath) and os.access(fpath, os.X_OK)
+
+
+    def name(self):
+        return self.__name
+
+
+    def resolve_self(self):
+        if self.__is_exe(self.__name):
+            return None
+
+        path_string = os.environ['PATH']
+        path_list = path_string.split(':')
+        for d in path_list:
+            exe_path = os.path.join(d, self.__name)
+            if self.__is_exe(exe_path):
+                return None
+
+        raise DependencyResolutionError(f'{self.parent_name()} depends on command `{self.__name}`, but the command cannot be found. Make sure that the named command exists, the containing directory is in $PATH if necessary, and the executable file has the correct permissions.')
+