[build] Refactor build options definitions

Split out build definition YAML files to allow different options based
on config, target, kind of module, and target/kind combination.

assets/build/target.boot.yaml

@@ -0,0 +1,27 @@
+---
+ld: clang++
+
+ccflags: [
+  "-nostdlib",
+  "-nodefaultlibs",
+  "-fno-builtin",
+
+  "-I${source_root}/external",
+  "--target=x86_64-unknown-windows",
+  "-ffreestanding",
+  "-mno-red-zone",
+  "-fshort-wchar",
+  "-fno-omit-frame-pointer",
+  "-ggdb",
+  "-g3" ]
+
+cxxflags: [ "-fno-exceptions", "-fno-rtti" ]
+
+ldflags: [
+  "--target=x86_64-unknown-windows",
+  "-nostdlib",
+  "-Wl,-entry:efi_main",
+  "-Wl,-subsystem:efi_application",
+  "-fuse-ld=lld-link",
+  "-g" ]
+