[build] Move configs/ to assets/build/

I was sick of `configs` colliding with `configure`, and these are
(mostly) static asset files anyway, they should live under assets/.

assets/build/boot-debug.yaml

@@ -0,0 +1,30 @@
+---
+extends: base
+
+variables:
+  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" ]
+