Add CHERI Alliance opensbi target.

This should support the standard zcheri ISA, support for the xcheri ISA is
uncertain and there is no hybrid ABI support.
We should merge the CHERI Alliance opensbi target with the existing opensbi target
as things converge.

Author: qwattash

pycheribuild/projects/cross/opensbi.py

@@ -208,3 +208,27 @@ def run_tests(self):
             give_tty_control=True,
             cwd="/",
         )
+
+
+class BuildAllianceOpenSBI(BuildOpenSBI):
+    target = "cheri-alliance-opensbi"
+    _default_install_dir_fn = ComputedDefaultValue(
+        function=lambda config, p: config.cheri_sdk_dir / "cheri-alliance-opensbi/riscv64",
+        as_string="$SDK_ROOT/cheri-alliance-opensbi/riscv64"
+    )
+    repository = GitRepository("https://github.com/CHERI-Alliance/opensbi",
+                               default_branch="codasip-cheri-riscv")
+    supported_architectures = (
+        CompilationTargets.FREESTANDING_RISCV64,
+        CompilationTargets.FREESTANDING_RISCV64_PURECAP,
+    )
+
+    @property
+    def all_platforms(self):
+        return ["generic"]
+
+    def compile(self, **kwargs):
+        for platform in self.all_platforms:
+            args = self.make_args.copy()
+            args.set(PLATFORM=platform)
+            self.run_make(parallel=False, cwd=self.source_dir, options=args)