Add missing files

startup.s

@@ -0,0 +1,15 @@
+// The .word directive allocates a 32-bit value in the memory. In this case it allocates in memory
+// the address of the stack_top (value in the linker.ld file) and _Reset.
+.word stack_top     // Address of the stack_top
+.word _start        // Address of the _start function
+
+// The thumb_func is used to make sure the function is in thumb mode,
+// which is required for the Cortex-M0+.
+.thumb_func
+.syntax unified
+
+.global _start
+
+_start:
+    BL main
+    B .