[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/user-debug.yaml

@@ -0,0 +1,38 @@
+---
+extends: base
+
+variables:
+  asflags: [ "-I${source_root}/src/kernel/" ]
+
+  ccflags: [
+    "--target=x86_64-jsix-elf",
+    "-fno-omit-frame-pointer",
+    "-fno-stack-protector",
+
+    "-g",
+
+    "-D__ELF__",
+    "-D__jsix__",
+    "-U__linux",
+    "-U__linux__",
+
+    "-isystem${source_root}/sysroot/include",
+    "-isystem${build_root}/include/libc",
+    "--sysroot='${source_root}/sysroot'" ]
+
+
+  cxxflags: [
+    "-fno-exceptions",
+    "-fno-rtti",
+    "-isystem${source_root}/sysroot/include/c++/v1" ]
+
+  ldflags: [
+    "-g",
+    "-Bstatic",
+    "--sysroot='${source_root}/sysroot'",
+    "-L", "${source_root}/sysroot/lib",
+    "-z", "separate-code",
+    "-lc++", "-lc++abi", "-lunwind",
+    "--no-dependent-libraries",
+  ]
+