Add basic boot code

It just prints a green `OK` to the screen and hangs.

src/arch/x86_64/boot.asm

@@ -0,0 +1,22 @@
+; Copyright 2015 Philipp Oppermann
+;
+; Licensed under the Apache License, Version 2.0 (the "License");
+; you may not use this file except in compliance with the License.
+; You may obtain a copy of the License at
+;
+;    http://www.apache.org/licenses/LICENSE-2.0
+;
+; Unless required by applicable law or agreed to in writing, software
+; distributed under the License is distributed on an "AS IS" BASIS,
+; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+; See the License for the specific language governing permissions and
+; limitations under the License.
+
+global start
+
+section .text
+bits 32
+start:
+    ; print `OK` to screen
+    mov dword [0xb8000], 0x2f4b2f4f
+    hlt